Co-parenting

I guess I need advice..

Backstory: my ex of 4 years and I have two daughters, 3+4. The basis of our relationship from the beginning was toxic, I was in a bad place mentally (after losing a child I wasn't ready for but chose to keep, and 2 days later, my grandfather who raised me died) and he was actively looking for one night stands but our fling turned into a month long fuck affair that resulted in our first born, 1 month after meeting I was pregnant, and 15 months after her birth I had a second daughter. He cheated through both pregnancies. I put up with it for a while but lost the will to continue the relationship because he was so verbally and mentally (even sexually on some days) abusive. I now have a wonderful fiance, he essentially saved me from the abuse and depressive state I was in from the abuse. My ex took my kids from me for a month for leaving him for my current man, I played nice, got them back, now he has biweekly visitation.. But I'm still stuck.

Now: this man loves his kids, but he also tends to favorite my eldest, and leaves the youngest to fend for herself for attention and love. I hate it, when they come back to my house my youngest acts up and the eldest brags about all the things he buys for her.. the icing on the cake was last month he eloped with the woman he claimed was his babysitter for like 3 months after we split, and then a month after he finally admitted she was more than my kids babysitter they dropped my kids off for my visitation, went to a resort without them or her son and got hitched, it really pissed me off he didn't include his kids.. i feel like even though he provides and cares about them, he uses them for his own benefit.. I guess I want to know if I'm being petty or if I should actually consider just taking them back full time and going for child support, I've tried to be nice but I feel like his instability is going to harm them in the long run.. and the fact that he's calling this new woman their step mother pisses me off too, because she may be his wife but she hasn't known my kids for more than 4 damn months.. help me out ladies.