Is it normal my sister dresses as a baby?

Is it normal that my 16 year old sister likes dressing up as a baby? A few weeks ago she was home "sick", I saw her dressed like one when I came home 1hr early from school. I heard her rummaging her stuff, so I decided I'd go and sneak in and scream in her ear to scare the shit out of her. When sneaking into the already ajar door, I stopped in my tracks because it sounded like she was getting changed, but I peaked in and saw she had covered her private area (safe to enter that means) but not as normal, she was wearing a diaper and from side on I saw she was sucking a pacifier. I was creeped out and ran to my bedroom.  Do all girls do this around this stage? Like she's always been known to be a different sort of girl, she currently has a shaved head (originally shaved for the Leukemia foundation but she decided to keep it that way to raise awareness or something like that, that's normal, most girls that age pretend to be social activists and stuff.) She's not disabled or mental ill or anything,   she's quite popular at school and if this got out I'm pretty sure I'd ruin her. That's why I'm asking here, anonymously. I'm concerned because she's always having sleep overs with this one guy, and out of curiosity last night I looked and she was dressed like it again outside my window going behind the hedges with her guy mate (who will remain unnamed). I didn't want to know what they were doing but it's safe to assume the worst as the rumor at school is that 'voldemort' sucked off a guy in the school toilets and gave two others hand at the same time, and she isnt denying it!! The first time I knew she was involved in this shit is when I walked in on her in nothing but a diaper sucking off, I thought nothing if it, I was 13 and she was 12. What's wrong with my sister? Is she going mad? Do I talk to her? Im creeped out and confused. Please help.