I spoke to DH and he reminded me that he has "grown up" in the role where BIL is constantly unavailable, so he feels duty bound to help her when she needs it as no one else will, and he has become very attached to the kids. He said that he will work with me to stop her monopolising everything, however.
OP - do you honestly not see how fucked up that is?
How about feeling "duty bound" to put his own fucking wife first? How lovely he has become attached to the children. Maybe he can adopt them? Okay, this isn't the children's fault, but at the same time, what the fuck is he going to do? Play the role of dad to two families? So his own child is only going to get half a father because he is too busy playing dad for his brother's children? what the actual fuck?
This is so overwhelmingly disrespectful to you that it isn't even funny. I have no idea why you put up with this. For real.
His brother's faults are not his sins to rectify. His brother's fuck ups aren't his responsibility to fix. If he wanted to play the role of "saviour" when he was a free agent that's one thing, but to continue to do so when he has a wife and child on the way absolutely begs belief.
Unless you see some serious changes pronto I would seriously advise you to go and spend some time with your own family. Do not accept coming second.
When you need your husband most, she will suddenly have some "drama" that requires your husband. What is he going to do? Tear himself in half? Clone himself? I would bet anything that he puts her needs first - partly out of habit, and partly because you've accepted it until now.
I could not be in this relationship.