Am I being a butthole?!

So, for a little bit of backstory. My dad passed away years ago and my mom is remarried. Step-dad has a daughter that is 10 years younger than me (From here forward she will be called SS for step-sister). SS is 21y/o and lives with my mom and her dad.

SS and I don’t talk. She’s honestly really, really strange. Like her presence actually makes me uncomfortable (Its not just me being judgmental or anything, literally anyone thats ever been around her feels creeped out after). We have talked maybe 5 sentences to each other in the last 6 years that she has been around. She locks herself in her room all day- to the point where I’ve actually only laid eyes on her maybe 20 times ever. She is more of a stranger that lives in my mom’s house than a step-sister.

On to the question... I’m 14 weeks pregnant. Husband and I have already told our moms, and are telling the rest of our families on Christmas. For our grandmothers, I made a couple onesies that said “Hello, Grandma!” and “Hello, Nana!” for them to open on Christmas. Our grandmothers are the only ones getting onesies.

My mom came up to me yesterday and said “You should make a onesie that says ‘Hello, Aunt SS’ for SS to open on Christmas.” I told her no, and she got annoyed.

I felt really uncomfortable about this, because

1. We don’t talk, so I do not feel the need to make her a special pregnancy announcement.

2. I do not intend to have my kids call her Aunt.

3. SS recently posted on social media that she hates children, so why would I make an effort when she’s not even going to like the kid?

Anyway, my mom mentioned it again and I just said “No.” she called me selfish. So I need you guys to tell me if I’m being a butthole.

Edit to add: For what its worth, my blood-sister thinks it would be super weird too lol.