Bathroom connecting to bedroom query!? my cousin lives in a house where none of the kids hold...

my cousin lives in a house where none of the kids hold their own bathrooms. there are 3 girls and they all share one to be exact on their floor. they built a guestroom that has its own bathroom but its far away from the rest of the bedrooms. she wants to "build her own bathroom" that connects to her room. she asked me if it be possible. i said she should just sleep in the guest room but shes determined to build a bathroom. is it possible for her to do this? i connote, i dont understand how she could create space if it isnt there!
.i choose best answer (:
Answers:    she can cut out sector of her own room and use that extra space for a bathroom but her room will probably be a lot smaller!
I understand completely why she wishes to stay near the kids- like within case of fires, or sneaking out at night. That is why I don't use my guest room at the other back of the house that is 24x24 feet. I too want to build my own bathroom, so I am good up for it...for someday. I will have to build out the side of my yard. It is possible if you hold the space or if you build on. It would be possible but not probable. You are talking thousands of dollars to add a room, reroute plumbing and sewer. I'm next to you, use the guest room.