<human>: I didn't tell my dad because he'd been trying to remarry and they'd ended disastrously every time. In my reasoning, he deserved to be happy. Plus, my sister had just been born then. I didn't want her growing up in a broken home. Stopped living with my dad 4 years ago and found out last week he caught her cheating. She promised she wouldn't do it again! I feel so stupid!!  Like it's all my fault. I've never had a worse week in my life!!! Now my sister is going to grow up in a broken home - along with my 3 half brothers(1 to 5 years)  I could have prevented this!

Condense the text into a few key points
<bot>: Caught my step-mom cheating, didnt do anything about it. Dad caught her cheating this time and i feel i could have nipped it in the bud