Not taboo in my opinion, it's not like their brother and sisters by blood. I mean like she's way older than him, I think thats about the only REAL problem. Except what their relatives will think...
its tabo cuz its her best friend brother which is like her lil bro has well considering she took care off him too , but i they should go for it cuz its better they already know each other hahah and it's sstaying in the family so to say !