My Step Sister Wants Tampons and I Wanted to Help..?

So my little step sister (12f) just started her very first period, and she texted me while I (19f) was at work today and asked if I could grab her some light and regular tampons. On my way home, I grabbed some from the store, and put them away in us girls’ bathroom.

Her dad—my step dad—found out about them and said he didn’t want her using them (“because it’ll cause her to tear”) even though I’ve used tampons all my life and have never torn down there. He wasn’t mad, and even said he appreciated the thought, but that me and my other biological sisters would be the only ones using them.

She has told me how she hates pads because they feel gross, uncomfortable, and like diapers. I also already told her that I bought them and she was so excited. I also really respect my step dad, see him like my own father, and don’t want to go behind his back. I’m really torn between the two.

Anyone have any advice?

Edit: I also feel really weird that he thinks it’s fine for me and my biological sisters to use tampons, but not his biological daughter? Like I don’t know if he sees us differently because we use those and menstrual cups…