My 17 year old Step Sister is scared of the dark still?

Anonymous
My new Step Sister has been living with me for just over a few months and I what I have noticed is that she is always scared of the dark and she has just turned 17. She's quite mature for her age and attends the same College as me. Just last night at around about 3am she came into my bedroom claiming that she was too scared sleep due to the dark and wanted to jump into bed with me - she did and it was weird she does this most nights and it's getting awkward now and too much.

How do I stop her doing this constantly because even though she's not my biological Sister it is still not right in my opinion considering her age to jump into my bed all the time when she is scared.

She doesn't have a lot of friends right now at College and after leaving school.
Updates
+1 y
Just mention unfortunately I don't have a lock on my bedroom door so my Step Sister can just walk in - that's the most annoying part of my bedroom.

She seems to phobia of other things also but this is the worst.
My 17 year old Step Sister is scared of the dark still?
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