Alright, I'm gonna have to jump in and kinda sit near the middle, leaning towards rapster.
On the one hand, when I was 19, I was dating a 16 year old (she even got her own post here). On the other hand, though I don't have a younger sister (though I have plenty of younger cousins), I understand the protective mentality of an older brother/relative.
Now, while dating the 16 year old, I never nailed her or anything and I had no intention of having sex with her anytime in the (then) near future (relationship got, ahh, cut short anyway). However, I would have fully understood if her brother, my age and in the military, wanted to beat the living **** out of me if I were. I'm sure I wouldn't have liked it though.
However, its in our nature to protect those close to us. And, even with our modern society, we still have some concern about the honor of a fair maiden. We also have the very real problem of STDs, which afflict younger people much more often then older people. And unintended pregnancies? Well, its those that brought my father, my sister, me, my nephew, my best friend's kid, etc. into the world. Plus, for many people there are some very real emotional issues with sex. And besides, I'm sure most of us, the moment we got into college, realized how incredibly stupid kids in high school are.
So, rapsta, first of all, I would say you should calm down. Second, look into this guy, make sure he's not complete trash. And make sure you got enough info to track him down if he screws over your sister (knocks her up and bails out of town for example). I'm sure none of us would hold it against you if you beat the **** out of him then, and I'm sure Sweden has some 'crime of passion' laws.
But, for proactive measures against this, you either
1) assault him physically
2) assault him and/or her verbally
3) sit down with her and explain your opinions
4) inform your parents, his parents
1 is a bad idea, could get you arrested, he might actually beat you.
2 is just gonna force them closer together, now that they've got you as a common enemy. Not good.
3 is unlikely to work, since most 16 year old girls (actually, most girls period) are headstrong and think they know better than you or anyone else. But, maybe you could convince your sister. Especially if you've been reading "How to Win Friends and Influence People."
4 is probably the best option, assuming the parents would feel the same way you do.
Other than that, there's not really much you could do. You could try looking up any possible legal action, sex between them might be ok, but there could be other stuff they're doing thats not. For example, maybe if they cross any borders, you could nail him for kidnapping, technically.
But think it all over, discuss it with a priest or someone, and don't do anything rash. It won't do you any good to have your sister resenting you for the rest of her life, cuz she'll probably **** around out of spite for you if she's like most women.