moral / ethical question

i am 20, 21 in a month. would it be morally / ethically wrong to drink with my 15 / 16 yr old sister and cousin ? not sure if this makes a difference or not, but i was definitely drinking at their age. they also drink, tell me about it, and both sets of parents are aware / okay with their drinking. not saying i would let them get blackout drunk or drink with them every week, but would drinking together sometimes be morally wrong ? they’ve also both already started asking me to buy them alc ( using their money lol ) once i’m 21. as far as legality, in my state “Parents or grandparents: Adults over 21 can provide alcohol to their own children and grandchildren on private property with parental consent.” so, although i am not either of their parents, if their parents were to provide the alcohol and we drank at either house it would be legal