IS anyone else gonna ask?!? Ok.. here goes, your brother lives with you, but his girlfriend, according to your words "doesn't even live 'there'" but the GF's daughter is trying like "hell to move in".
What sort of situations are transpiring if your brother's girlfriend doesn't co-habitat with you, but her 19-year old daughter is showering in your place?:eek
I can just picture, "Hey... I'm just going to shower her for no reason, and maybe hang around uninvited and sleep here too".