I just wanted to say I think you are doing the right thing. To me, it seems you are the one responsible and caring for the children, providing for them and being there for them. It is way to early in the pregnancy for her to tell them. I do feel they should be told about their half sibling either later on in the pregnancy or after the baby is born. The kids will have no concept of time and if something happens, it would be another heartbreak for the children.
If the mother actually does become a part of your childrens life then you will need to accept this child as their sibling. It does not sound like the mother will to me though. It sounds like she cannot deal with commitment and responsibility and will probably abandon this child as well when she tires of it, which is very sad because it is the children hurting in the end, no one else.
Please protect your children with what you believe is right for them. My ex husband was kind of the same. He would say he was coming to get them, the boys would be packed and waiting by the door and he would never show. That was so hard on the boys. No matter my feelings for him I did encourage their father to be with them because they did love him but it got to the point I had to do what was best for them. If he would call to say he wanted to see them, I wouldn't discourage it but I never told the boys he was coming so they would not be hurt again if he didn't show. Eventually, he forgot about them and they have not heard from him since they were 12 and the oldest is now 28. He never paid support and I never got the $100 yet he said he mailed in 1992. LOL My current husband of 20 years was their dad. He has been in their life since they were 2 and 3 and they consider him their dad.
We have a blended family of his, mine and ours and all those kids call each other brother and sister. All 7 of them! LOL I would not discourage that if there is a new baby coming.
I guess I wanted to say, I think you are right in what you did and to protect your kids from emotional and physical harm by all means that you can! Encourage a relationship with their mother but if it is going to hurt them more than help them, protect them from her. It seems you are their only means of stability and love in their life. I commend you for sticking by them and doing what you feel is best!
Good Luck and hang in there. Things will get better!