roommate uses my bathroom

hi, i’m 19F living in a 2 bed 2 bath apartment with 4 roommates. 2 of my roommates share 1 bedroom/bathroom (master bedroom) and me and the other share the other bedroom/bathroom. as of recently the 2 in the master bedroom have been having problems with each other and one decides that she wants to “move” her belongings into my bathroom. her stuff is overtaking my already shared space, and it sucks considering they have so much more room than us. she’s always in my bathroom taking 1-2 hours in it before she heads to school which is annoying. how should i go about this? how should i bring it up.