why now advice needed

so..... me and my ex have 3 kids together. he has 3 to his other ex (now girlfriend) there 3 are 7, 6 and 9months. and our 3 our 2, 9 months and im currently pregnant. yep. messed up situation. but hes back with his ex anyway. so long story short. his kids go to the same school as my 7yr old son from a previous relationship. so everyday we see eachother. her two soms are of course aware of there 2 yr old sister. but yet. at the school she would normally never let them come near her. she would pull thwm away by the arm. but he has now walked away on my kids. to be with her. and hasnt seen them in 11 weeks. but yet SUDDENLY now she lets her sons come and approach me at the school to play with her and ask me when shes going to grannys. etc. why is she suddenly ok with this now. knowing hes walked away. and that will only mess there heads up more. im confused. and what do i do. becauss if he wont step up. i need for my 2yr old to forget them. not this constant reminder.