Am I wrong?

I asked my mom and little sister if they wanted to go eat lunch with me today. She asked if I’d invite my step sister too so she didn’t feel left out. But I said no, because I really just wanted to spend time with my mom and sister. Well, my mom got mad that I didn’t invite her and apparently her feelings are hurt. Heres the thing, I don’t mind my step sister. But, my mom has only been with her husband for like 2 years now, and my step sister just started coming around a few months ago. I don’t know her well, it’s not like we grew up together and have a sisterly bond. And every time I’ve seen my mom and sister for the past few months, she’s been there bc she’s living with them. Ive been trying to get to know her and be nice to her. She’s pregnant with her first child and the bd dipped. So I gave her a lot of my sons baby stuff, I’ve bought stuff for the baby, I helped her get into a OB, Ive been trying to be as helpful as I can. But, I really just want to spend some time with my mom and my little sister. I texted my mom, it’s not like I asked only my mom and sister right in front of her and didn’t invite her. What’s wrong with that?