Embarrassed By Older Sisters University Antics

So im 25 and been working since I left university. So has my older sister who is 33 as a teacher but has re-entered as a postgraduate study. However it seems she's changed. Since meeting this really nerdy first year student who is 18 she's hung out more with them since meeting them despite being a mature married woman with children and after me personally not liking her new friends from finding them strange I checked out their Snapchat stories from recent parties, and one story came up where it showed a dark car and a caption saying, my first blowjob. There was my sister dressed in bra and panties showing off her huge ass sucking off this nerdy first year in his car til he came in her mouth! The weirdest one came when she was in the room of another letting this weird nerdy guy tickle her big feet Coz apparently that gets him off.
She's not progressed to full on sex yet but her being unfaithful to my brother in law whom the family loves after she re entered university is embarrassing and I dont get what's caused this change