To begin with, I am 22-year-old girl who studies far away from home town for 4 years by now. All my family lives in Russia, if this is important.
I have 13-year-old sister whose relations with our parents are not that good. To understand the situation better, the dad behaves like he's only interested in TV and occasionally our school marks; the mom is more emotional, but often gets captious towards my sister.
One more problem is that I was a "good child" (excellent marks for every subject + afraid of parents scolding me so very obedient). My sister is not that good at school and more rebellious, perhaps because everyone used to tell her how good I was in comparison. So she often gets scolded.
We are not very close with her, just chatting from time to time.
Earlier this year, I noticed on social media that she seemed to keep company (more probably, she had a crush on him) with a boy whose age I don't know but he seemed no less than 16 year old. There were some... objectionable groups on his list -- with vulgar pictures of "loli", anime style preteen girls. Then one day she asks me to block him (we never talked about him before, he doesn't know me) because "they had a quarrel so he can send me some slander about her", maybe fake nudes or so. I told her whatever he sends it's him being disgusting, not her, and parents won't know and I won't shame her even if there were real nudes. The guy never contacted me.
Then soon after this story she meets a guy in her school. She doesn't tell parents but mom sees them walking together near our house. She says he's 16 but his social network page says he's 18 (usually in Russia school graduates are 17-18 year old), though he of course can put fake info on his page to look elder. He looks older than 16, but who knows. She denies having him as boyfriend but she changed her last name on her page to the same as his, and her profile pic is them taking selfie. Well, not going into details, it's obvious by her page that she at least has a crush on him or they are a couple.
Then one day dad sees them just staying together on stairs next to our apartment door (in Russia, residents usually are unfriendly and suspicious to anyone spending time there, almost no one does it) and yells at the guy, telling him that he shouldn't be seen there anymore. It's the first time dad sees the guy. Dad always yells a lot, of which I am afraid but my sister isn't.
Then one day mom comes home earlier than usual and sister doesn't open the apartment door (so mom has to wait on stairs) for some minutes which is pretty uncommon. When she eventually comes in everything seems ok but the restroom door (very close to apartment door) is closed. She proceeds to the kitchen and hears the apartment door closing. Clearly my sister had a guest she didn't want mom to see. Mom decides not to talk about it with my sister but calls me the next day being worried that my sister had sex with that guy.
Then sister tells mom the teachers grumble about her having "just a friend" elder than her. And this guy also has "loli" groups. And my sister too. Perhaps it's popular in teens nowadays.
Then mom gets a sneak peek of sister's messages (I think it's absolutely wrong, but it was mom not me) and sees something about this guy having problems with his sleep due to some drugs (for context -- "light" drugs like Spice or naswar are sometimes heard to be used among "bad" teens in Russia; drugs are illegal here but sellers often hire teens as couriers). I don't know the exact text mom has seen. Supposedly my sister doesn't know mom saw her messages.
So what should I do now?
1) Can I somehow discourage her from having affairs with older guys at that age? I've heard it's common for teen girls to like boys older than them, but I feel that 16-year-old boy being interested in 13-year-old girl is creepy. My sister is definitely not too mature for her age, neither in mind (not the kind of a kid who's bored with coevals) nor in body (very slender, tiny breasts). What I'm afraid of are "unequal" relations when he can press on her "being elder and thus knowing better".
What makes things a bit worse, I have a boyfriend since I was 14 (he was my classmate), so if I tell her something like "maybe it's not the time" it may sound like some double standards (even if it's not, she's 13 and having a crush).
2) Should I hurry to tell her about contraception? My parents think that there's no need to tell children about sex (it's quite common in Russia: they told me nothing and always avoided the theme, so it was me who told my sister about menstruation). No sex ed in school also. Now I feel I have to tell her about contraception but we're not going to meet in person until summer at least. Before that I can visit them for a week at most, it's expensive but I have the money. Mom won't ever tell sister those things (I asked). Moreover, I'm probably the only relative who can ever say to her she can have sex before marriage and even have an abortion if she doesn't want the baby.
Or is it too late to do anything? I guess, for instance, if I tell her withdrawal is not safe at all but the guy tells her it's safe and he doesn't want condoms, she will listen to him, not me. And so on.
TLDR 13-year-old sister seems to have 16-year-old boyfriend. Can and should I do something?
Obviously it shouldn't be my problem, but the parents prefer to either be angry at her for keeping company with any boy or pretend nothing happens (that's their strategy when she has problems in school). But I feel things aren't alright and it's wrong for me to just look. Or am I wrong?
P.S. Feel free to ask if something is unclear: my English is not so good.