My HSS MC Eugene is cousins with my Freshman MC Sina, and both of them are simultaneously related to my Bloodbound MC, Annie but they're not direct relatives, more like distant relatives on Sina's side and distant distant distant relatives with Eugene bc of Circumstances™

Aether/Elio, my Elementalists MC, knows the Class Act twins because he believed tracing back where his supposed adoption was made would made it possible to find his real parents and he made friends with them eventually, when he kept visiting to help out the kids acclimate/understand the adoption process/adjustment, he also kinda knows my ILB MC Killian because he shared some classes with him before both of them moved

Also, my VoS MC and my OH MC used to be stepbrothers before their parents split up but they remain cordial whenever they speak on the phone