NTA.

I have a biological son and a stepson who are very close in age. I can't fathom ever doing this or my husband doing it either. When I married my husband, that meant that all of our kids because our kids. If anything like this did ever happen, that would be one helluva argument after the kids were out of earshot.

Talk to your father. Make it clear that it isn't about ice cream, since he might blow you off, thinking you shouldn't care so much about something as "trivial" as ice cream. Make it VERY clear that it is about her not accepting you as family, treating you as an outsider, and deliberately excluding you from a treat, forcing you to sit there watching others indulge while you go without.

Also, WTF to your stepsiblings. The fact that they didn't say anything shows that she is teaching them to see you as "other" too, which is fucked up.