In need of some serious advice

Ok I am a mother of three boys , and currently 7 weeks pregnant. I have a sister who is 13 months younger then me who had no children of her own, we have never really gotten along but through the years she has helped when she can and has especially gotten close to my middle son who is 4, to be honest she is like a second mother to him and fills the gaps on things that I can't , like taking him places I can't afford of throwing him his 4th birthday party back in July money was really tight for me and my fiancé then and often is. But every chance she gets she finds a way to slip in how bad of a mother she thinks I am , mostly because I'm young and that I can't afford all of my children's wants !! Believe me they get everything they need! Well last Saturday we exchanged words and she got physical mind you she didn't know I was pregnant even though I'm not sure it would have mattered if she did. I told myself that I wasn't going to talk to her anymore since her bashing me as a mother is constant , but my four year old keeps asking can he go see his Annie he is innocent in this all and she is someone he loves very dearly and it's a relationship I don't want to rip away from him just because of our issues.well I gave in today and texted her that he wanted to talk she texted back have him call me , she only spoke to him and only spoke to me through him and asked could my other sister pick him up tomorrow so she could take him to the movies, I am fine with this like I said I don't want their relationship ruined because ours is , but fiancé says he shouldn't be allowed to go with her unless she apologizes which I can understand but I'm never gonna get that out of her and my son is already excited about going so what am I to do ? Let him go and just not tell my fiancé ? Or keep him home and tell her I think we need to talk about boundaries before she can take him again ? Please help idk what to do