Hurtful as this may sound, you don't get to choose. Because you pushed your children's father away, he has now moved on and you don't get to choose. He gets to choose who he wants to be in relationship with and you get to deal with the fall out. It doesn't matter what you think, he had every right to have her there at their first communion.
If you really love him, stop playing the games. Love is precious and live your last days in love. Love takes care of those whom you love whether they are sick or well. Yes it would be tough on him and tough on you and tough on the boys but you will have each other. I agree with others who encourage you to get counseling.
Your kids probably don't like her because you don't like her. Even if you never say anything, body language speaks volumes too.
I'm sorry you are going through the battle of a life time and I'm sorry you don't have the man you love by your side but you did this to yourself, so don't blame him for moving on in a direction that doesn't meet your approval. You should be honest with him and try to spend your last days in harmony and at peace because you don't get to choose his life for him.
Ultimately wish him well in what every choices he makes and be positive instead of negative because this is best for your kids.
Also understand, kids that fail may or may not be the input of the parents, we all must take responsibility for our own actions. Her kids are responsible for their lot in life and the choices they have made. Perhaps she is a very good mother with very bad kids. It does happen. You don't know because you have drawn a conclusion from the facts on hand.