How old should a baby be potty trained??

I currently have a sister who’s 2 and a half and I really believe she should be potty trained by now but I don’t want to say anything since I’m not the mother ( she’s my step sister, step mom daughter )

I have to watch her and I feel uncomfortable changing her diaper as when lifting her legs as they reach the top of my head.

I mention that I don’t want to change her diaper anymore and should at least be using pull up diapers but my dad said just don’t do it and she’ll learn eventually. That it’s up to there mom and that they’ll learn

I mention this slightly to my boyfriend mom and she trained her kids by the time they were 12-15 months old. Also that they should be using pull up if they’re that old otherwise it’s bad parenting to still be having to change there diaper when they’re old ( she’s from Vietnam and she said it’s important to have/teach them to be independent young )

I just think in my opinion she’s to old to be using diapers but I want y’all opinion on this. Maybe I’m in the wrong 🤷‍♀️ maybe I’m not 🤷‍♀️