Whether your partner is step farther, or biological farther, or their yours, his, or the bloody postman’s is absolutely irrelevant. You are a couple, and together you raise your children. They are siblings and should be treated as such. She invites all or she invites none.
Personally I’d tell her very firmly that she can not pick and chose which of OUR children she invites. It’s all or nothing. And unless I got a massive apology and it was some kind of accidental oversight I wouldn’t attend. If I did attend, I certainly wouldn’t be bringing 40% of my kids whilst leaving the rest at home!
I have 2 children, 1 is not my partners and 1 is. My oldest still sees his biological dad alternate weekends, and we get on fine, however my partner is an equal parent to him, he’s the one that raises him with me 12 days out of 14, pays for him, and keeps a roof over his head. Both children are treated exactly the same, my partners parents are both children’s grandparents, and my partners sister is both children’s aunty.