Have to share my bedroom with my 18 year old sister :(?

my parents are getting a loft conversion and we wont be able to use my sisters room for about 3 months. I had to share with my little sister a while back and it was hell. I like having my own space i have to share my bedroom AND A BED!!! but my mom just doesn't seem to get that i want my own space and bed and my dad has loooaaads of brothers and sisters and said how they had to share rooms, thing is i have like four exams coming up and how am i supposed to sleep and revise when my sister (gap year from university to work) is coming in at like 4am from clubs and getting back late from work? anytime i try to tell her this she scoffs at me and they have really high hopes for me i want to do medicine but how the hell am i supposed to get A*s at gcse when i cant sleep properly please help