Honestly, I'd be wondering if that's the only/main reason they're visiting you. Quite odd for someone so young to travel to stay with another relative with boyfriend/girlfriend.
"At 16 you can get married." In some countries. Not all by any means.
My dd is 17 and sexually active when she chooses and I'm fine with that. BUT I'm her parent so it IS my choice particularly wrt whether she has boyfriends stay over. As an aunt no I don't feel it's in your remit to decide this is ok for someone else's child. Particularly the boyfriend who I'm guessing you don't even know? Or certainly don't know well. Boys can be put under pressure too.
In addition - they know her, and probably the boyfriend better than you. They may be aware/concerned about pressure he's putting on her to become sexually active before she's ready. You have no way of knowing if that's the case.
You do need to be honest with bil that you have neither the will nor the ability to police 24/7, you've enough on your plate by sounds of things.
"I care about my niece and dont want her doing the same." Completely irrelevant post, STILL not YOUR child.
"There are very few countries with an age of consent above 16 years of age." You think so?
How about
Ireland
Cyprus
20 of the USA states
South Australia and Tasmania
25 African countries
4 Asian countries... Not uncommon at all actually.
"I looked online and the age of consent where they are is actually 15."
BUT still not an adult and doesn't negate the fact that you're not her parent AND that he may have concerns she's being pressured/coerced.
"I would not host somone else’s child eg the boyfriend what if they split up or have a row when their over not mature enough in my view to be travelling together" I'd be very uncomfortable with this too. It's VERY young to be travelling as a couple and I'd also be concerned about potential fallings out. Even for older couples going on holiday together can be a real test of the relationship, especially if not yet living together.
I agree with corythatwas going behind bil's back would be disastrous for family relationships all round and sets a bad example to your niece.
Regarding "not in Scotland" that's often said because it's often assumed that English/Welsh law is uk wide and it isn't. Right from the beginning of the thread the answers in terms of legality all assumed
A niece from England/Wales
B op living in England/Wales
When there are mners and their families all over the world.
"Tbh there isnt any relationship, the limited contact we have at the moment is purely for our nieces and nephews." Ahh now we're getting to it - you don't like bil so that's why you thought it remotely acceptable to go behind his back - not cool op.