Wow. You said it very nicely and so true. I'm in the same situation but the kids love me and they have no issue so far. I feel the jealousy and seeking for attention from his daughter sometimes. However, I have issue with not being number one. It's not an easy task.
Actually, I think you are very wrong of saying the kids should come first. Even bible doesn't agree with you. Parents come first. If the adult relationship is not good together then kids will suffer the most. It's like when we fly, they teach us take care of yourself first and put the oxygen on yourself first before helping any minors. Life is the same, if the adults don't have good loving relationship, then they can't be good to the kids either.