(((Hollie123)))
I am very sorry that your bf and sister had a fling. I can only imagine how that feels for you.

Your bf sounds like a real gem.

Having kids together definitely complicates things a lot.
I would recommend becoming a lot more insistent with your bf. Like: "We really need to go to a counselor NOW. We need to build a healthier relationship ~ our kids deserve better than what we're giving them right now. Us standing together is real important for our kids to feel safe and loved by us." If your bf is unwilling to go as a couple, go by yourself and work on building strength.
Keep going to school, and get that career.

You don't always have to depend on others to keep you going. That education can help boost your self-esteem and income.
Hopefully, you and your sister will be able to be honest with one another about your feelings. Wish you the best!