You will adjust. You'll always hate it but the shock will wear off. I would consider IC to get you through this transitional phase. Adjusting to your new 'normal'.
I went through it with knowing my precious little girls were around here. It took a few months but the shock and horror wore off. I still loathe it but its not a burning, heaving feeling anymore.
I've been lucky in that I haven't accidentally seen either of those whores even though he and I live quite close to each other (praise the stars!).
I think I'd laugh out loud if I did see them together - they must look ridiculous, people assume she is his surly teenager. .
Your youngest daughter is probably enjoying all of the attention he is raining down on her right now. She doesn't realise she's being used as a pawn to validate what a great dad he is - y'know, the dad whose oldest daughter won't have anything to do with him? Yeah, that's the one. Its intoxicating - I hope she doesn't get burned. But it is something she'll need to learn for herself.