Coparenting advice please

Hey all, This is not a personal question, but it does involve my step daughter and her siblings. Her mother and father divorced and afterwards and quickly, the mom had twins. She is not a good mother, flat out. My boyfriends daughter lives with us now and it’s much healthier, happier, peaceful and consistent. Her mother has not seen her other two children (not with their father either) since October 2021. Has not texted, FaceTimed, reached out or took any initiative at all.

She just recently saw her other two children a few weeks ago and I’m so happy because that means now my boyfriends daughter can slowly reconnect and be in her step siblings lives (she misses them terribly.) Every time she asked her mom to see them, her mom just gave poor excuses and blatant lies (there’s proof of attempted correspondence from the twins father, emails and messages were shown when boyfriend was finally able to reach out and get in touch with him.) All of her lies and excuses came to ahead and the twins father was always nice, civil, he just wanted to know what was going on because the twins miss her a lot.)

Anyway, me and my boyfriend had a little debate. He feels that told the first visit that his ex wife had with the twins (since October) should have also involved his daughter (their step sister.) I feel that what she did was ideal and that those two children should reconnect and attempt to establish a relationship with their mom alone. I never said not to take her to see siblings, but I don’t think she was wrong in this case and he does (he feels his daughter got slighted.) The twins are five and they do have a little resentment because they are hurt and can’t understand why their mommy won’t see them but has seen their sister (my boyfriends daughter.)

My boyfriend got nasty and took it as an insult to his daughter and told me all he cares about is his daughter (which is beautiful that they have a very special bond) however, he’s treating my comment as some sort of attack on his daughter and that’s not the case at all. I absolutely care about her feelings, but I do feel for the twins as well. I feel like it was the right thing for his ex wife to go see them by herself for the first time in months, give them some of her undivided time and re build their trust for her and overall relationship in general.

Question, or maybe just asking for your opinions on this matter and what you all feel. Thanks everyone