13 year old and 17 year old??!

Question

My sister is 13 and recently I found out that she's got a 17 year old boyfriend.
My first question was what the hell does a 17 year old want with a 13 year old? Given the maturity difference I know it's not a serious relationship.
Then yesterday I found out that they've been planning on having sex, which in the UK is very illegal as he's past the age of consent, but she is very much below it.
I really don't know what to do because he's just going to end up hurting her.
And I really don't need lecturing how it's her decision because she's so immature and doesn't fully understand the consequences.
EDIT - her mum is allowing it to happen, so my parents (she's my stepsister) won't get involved.

Answer

Okay so update
He cheated on her and now they've broken up
I'm angry that he cheated but I'm also glad she got out before anything happened

Answer

You need to bring a parent into it. You need to help her.

Answer

She's to young do something if no one helps you take matters into your own hand she's your half sister you still have to take care she can't have sex she has to be at least 15-16

Answer

It don't matter what part of the country your in .. That is wrong on all level on insanity... You need to get some one involve if not legally this is illegal and wrong she's not even ready emotionally mentally physically ready for that type of relationship yet, If anything that 17 year old is a pedophile and need to stay away how can he look on a 13 year old and sexualize her wanting to have sex she is just a baby .. The cops need to get involved tbh if u don't want that to happen for what ever reason try the very hardest to keep him away from ur sister let ur sister know that ur protecting her from  far more than what she think it is RN .. Let her know that the day when she decide to give her most prize possession away at this young Age to that pedophile shell never get it back she doesn't know what love is yet explain to her drill her get her get him get them away from each other...

Answer

I've tried getting parents involved but because her mum's allowing it, her dad and my mum won't step in and do anything.

Answer

Try and tell her yourself. I'm sorry, but a 17 yo and a 13 yo is a bit too young... She doesn't know anything about sex and he might be taking her to his advantage

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Tell her parents & if they won't do nothing about it then she's just gonna have to suffer the consequences.

Answer

You need to talk to her about it, or maybe even bring a parent into it. It's very odd that a 17 year old wants anything to do with a 13 year old, especially sexually.

Answer

please stop her

13 year old and 17 year old??!

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