Get a GOOD lawyer. State keeping a "List of Events" sheet. Write down who the kids are with. As for your stepson, he may be his but if he left him in your care you may be able to get custody of him IDK. If he hasn't spent time with him then the courts may rule in your favor for custody. That's why you need a good lawyer. Get online search out every thing you can on the matter. Document the lies, the drinking, the GF's behavior, EVERYTHING...they say that moving in with someone can cost you custody well the same should go for a man(?) Remind the kids of how much you love them, even seek counseling for you and the kids.
You wrote
"I am so afraid that after the divorce he will use the kids, hurt them (emotionally), and I won't be able to do anything about it."
There is something you can do. You are their mother ( blood or not) and as mother's we fight until our last breath for our kids. Do your homework and you will win. Maybe one of the woman here have more info for you on stepchildren. Good luck
A friend of mine has custody of...let me see how to explain it...she was living with a man whose daughter had a baby. The baby was a drug baby with special needs. The Grandpa and my friend were caring for this child. He moved out (OK, so she kicked him out)and she has custody of the little girl. She had cared for the child since she was a baby and was able to prove she were the most fit parent. DOCUMENT, DOCUMENT, DOCUMENT. Lisa had good ideas as well, and she's right-fight to the end. Hang in there. Prayers and hugs.
i really know how you feel . i been having sex with my ex to because i don't have any friends and no family support ...even though my ex will treat me badly emotionally afterwards. all that we can do at this time is try to be strong and break this bad pattern and get our self esteem back so we no longer allow ourselves to be used. As for the other woman it sounds like your ex and her deserve each other , you deserve better but i know how much it hurts.
If that woman is abusing her children maybe you should talk to a lawyer to see if what you can do to keep the children from that sitution for thier best interest.