What do i do?

So, I have a younger sister (14F, Freshman) "Liz". Liz used to go to group therapy with this one dude, (16FtM, Sophomore) "Asher". She actually used to date him for like a week until Asher broke up with her because he already had a boyfriend. So yeah, I don't see him as a good influence.

He keeps wanting to have Liz sleep over at his house, which my parents would allow since he was "born a girl" and they let anyone who can't get my sisters pregnant (for some reason) have sleepovers. Anyway, Asher keeps trying to offer Liz weed and whiskey/vodka. Of course, she's curious, but I really don't like how Asher seems to be and I don't know if I should set something up with our parents to not have her come over to Asher's. Or should I let her experiment and try it and not say anything despite how I feel?