Wow that's a difficult one isn't it, how lovely are you two. Lucky kittens finding someone who cares.
I would talk to her, is she approachable? what age are her children.
She may be finding it a bit hard & you could be the help she need's, & it's not as if you are miles away if the kiddies want to see the kittens.
Just as long as you don't get used for paying all the bill's then her wanting the kittens.
I would have a word, tell her what's been happening & what you have done for the kittens.
Let us know how you get on....... looks like you both have fallen head over heels anyway