My Sister Slept With My Fiancé To “Get Back At Me” For Being The Favorite Child
This story is a devastating look into what happens when family favoritism poisons every relationship it touches. The original poster (25F) grew up as the “favored child” after her mother’s miscarriage, while her sister (27F) slowly turned into the overlooked one. What started as emotional neglect spiraled into lifelong resentment — and finally exploded when the sister did the unthinkable.
After years of trying to mend their broken bond, OP thought things were finally improving. She had a steady relationship with her fiancé, John, and even introduced him to her family. But behind her back, her sister and fiancé had been having a secret affair that lasted for years — one that ended in pregnancy, confession, and complete heartbreak.
Now OP is left shattered, dealing with betrayal from both the person she loved and the sister she once admired. This isn’t just a cheating story — it’s about emotional manipulation, generational trauma, and the lifelong scars of toxic parenting.
In some fringe cases, parents playing favorites can lead to such intense sibling rivalry that it upends people’s entire lives
Case in point, one woman revealed how her envious older sister completely wrecked her romantic relationship
Okay, let’s be real for a second — this whole situation is straight out of an emotional horror story. It’s not just about cheating. It’s about family trauma, narcissistic parenting, and the deep resentment that grows when one child becomes the “golden one.” When parents play favorites, they don’t just ruin sibling relationships — they create lifelong emotional chaos that bleeds into everything, from love life to self-worth.
Now, let’s break this down piece by piece.
🧠 The Root: Family Favoritism and Emotional Scars
Family favoritism is more common than most people realize. Studies from the American Psychological Association (APA) show that around 65% of families have a noticeable level of favoritism. It usually happens when parents project their hopes or fears onto one child. In this story, the OP became the “miracle baby” after a miscarriage — a symbol of healing and joy for her parents. The problem? Her sister instantly became invisible.
This kind of emotional dynamic breeds resentment. The “favored child” often feels guilty and pressured to stay perfect, while the “forgotten child” feels angry, unloved, and desperate for validation. It’s a lose-lose situation — both end up emotionally damaged. And you can clearly see that dynamic in how OP tried to reach out, while her sister pushed her away. It’s classic scapegoat vs. golden child behavior, a well-known pattern in toxic family systems.
According to Dr. Karyl McBride, author of Will I Ever Be Good Enough?, the scapegoated sibling often grows up feeling like “nothing they do will ever be right.” That pain doesn’t just vanish — it mutates. Sometimes, that turns into self-destructive behavior; other times, it turns into revenge.
💔 Betrayal and the Psychology of Revenge
Let’s talk about the sister’s actions for a second — sleeping with your sibling’s fiancé and getting pregnant is more than just betrayal; it’s psychological warfare. She wanted to cause maximum pain. Her exact words — “Now you know how it feels to get your whole life ripped away” — show that this wasn’t about love. It was about punishment.
Psychologists call this “displaced retaliation.” When someone can’t punish the true source of their pain (in this case, their parents), they redirect that anger to a safer or more available target (her sister). It’s twisted but deeply human.
Cheating in relationships already causes enormous emotional distress, but when the betrayal comes from both your partner and a family member, it becomes trauma-level pain. There’s a loss of trust in everyone — your family, your partner, even yourself. You start asking: What did I miss? Was I not enough? Did I deserve this? These thoughts are part of what’s known as “betrayal trauma,” a term first coined by psychologist Jennifer Freyd. It’s the kind of trauma that happens when someone you rely on for emotional safety becomes the one who hurts you the most.
🩹 Healing After Betrayal
Now, let’s shift from heartbreak to healing. OP’s life isn’t over — far from it. But she’s definitely in what therapists call the “acute grief phase.” It’s the time when your brain is flooded with cortisol, you can’t sleep, you replay the betrayal over and over. That’s normal. But it’s also dangerous if you stay stuck there too long.
A few proven coping strategies could genuinely help:
- No Contact: OP already blocked her sister, which is good. She should do the same with her ex. Constant reminders (texts, pictures, social media stalking) only reopen the wound.
- Therapy for Betrayal Trauma: Finding a therapist specializing in relationship trauma or toxic family dynamics could help her process everything. Online therapy platforms like BetterHelp or Talkspace are great for people who work remotely.
- Rebuilding Self-Esteem: When betrayal happens, self-worth takes the biggest hit. Small wins — exercising, journaling, reconnecting with friends — help rebuild that identity brick by brick.
- Avoiding Toxic Parental Validation: It’s understandable that OP doesn’t want her parents’ support. She’s right — they created this mess. But she should still tell them the truth, not for approval, but for closure.
Healing won’t be instant, but it starts with recognizing that she did nothing wrong. Her sister’s choices came from bitterness and unresolved trauma, not anything OP did.
💬 The Role of Parental Responsibility
It’s wild that the parents haven’t been told yet, but let’s face it — they’re a big reason this all happened. When you emotionally abandon one child and overprotect another, you create two wounded adults. The “golden child” grows up anxious and guilty, while the “scapegoat” grows up resentful and vengeful. This dynamic is backed by decades of family psychology research.
In fact, family therapist Dr. Susan Forward, in her book Toxic Parents, explains that favoritism “splits siblings into roles that keep them from ever knowing who they really are.” The parents in this story never fixed their favoritism, and their daughters paid the price — one through guilt, the other through rage.
If the parents ever find out, they might defend OP, but that won’t solve anything. The real closure would come from both sisters cutting off their toxic family system entirely. Sometimes peace comes not from forgiveness, but from distance.
🧩 When the Pain Feels Unbearable
OP mentioned having alarming thoughts — and that’s a massive red flag. Emotional trauma from betrayal can lead to depression, anxiety, and even suicidal ideation. If you or anyone feels like that, please reach out for help. Call your country’s local mental health hotline or talk to a therapist immediately. You don’t have to go through this alone.
Also, journaling about anger (without judgment) can help the brain process complex emotions. Writing letters you’ll never send — to your sister, to your ex, to your parents — can be surprisingly healing.
💎 The Long Game: Rebuilding Life After Betrayal
Here’s the truth — five years from now, OP won’t even recognize this version of herself. She’ll look back and see this as the moment her real independence began. When everything burns down, you get to rebuild your life your way. No family expectations, no codependency, no manipulative partner. Just you.
Healing from betrayal is like re-learning trust. It starts with trusting yourself again — trusting your gut, your boundaries, your worth.
Many people were utterly shocked by the family drama. Here’s their perspective
This isn’t a story about just a cheating fiancé or a toxic sister. It’s about the systemic damage that favoritism and emotional neglect can cause across generations. OP didn’t deserve any of this, and her pain is valid. But she also has a chance now — a painful, raw, but powerful chance — to build a new life where she finally gets to be free of all that family toxicity.
If there’s any lesson here, it’s this: you can’t fix broken people who refuse to heal. You can only heal yourself — and that’s exactly what she needs to start doing.