@Dave20 As several posters have said, the vast VAST majority of unemployed 18 year olds do NOT have their own place. Why does your wife's niece think she should be entitled to this, at YOUR expense?
A number of posters have said they would not do it for anyone but their DC. We would not even do it for them. We were in a precarious financial position when our first DC started at uni - almost 10 years ago - and had a low income, and they shared the uni house with 5 others and were jointly and severally liable.
There's no way we could have forked out multiple 1000s of pounds if DC or any of their mates had defaulted. Fortunately, because they were the only one of 6 without a guarantor (a bunch of 6 friends,) the landlady let her in with no guarantor.
Turns out it was a good choice. She got excited at having some money to herself (student loan) and spent frivolously, and ran out of money quickly. She's an intelligent hard working, responsible young woman now, in her late 20s,) and is good with money now, but at 18, she was a bit reckless.
So long story short, 4 months into her degree (by the January,) she'd only paid one month's rent. She was on the phone crying and admitted she had no money left. We told her we weren't paying it as we simply didn't have the money (at the time.) We said 'you need to sort this out with your landlady...'
Luckily, the landlady was a lady with 7 properties that she rented out to students, and although she wanted the money, as it was her business, she was OK to wait a few months, and was very good and very lenient. By July, DD had caught up and was fully paid up!
It taught her a lesson, as we didn't/couldn't bail her out. (Probably wouldn't if we could have!) After that, she became more responsible with money.
Also, if this had been a strict, not so laidback landlord/lady, and we HAD been her guarantor, I have absolutely no doubt whatsover that they'd have been after us for the £1500 our DD owed.
Some posts on here are terrifying, like the poster who said she is a guarantor for someone who's buggared off and left the agreement and she owes six thousand pounds already. And the official agreement hasn't even taken place yet. Could be nine to ten thousand yet.
Even now, (being in a better financial position,) we STILL wouldn't be a guarantor. As has been said, it's basically an open-ended agreement with no limit. By the time someone is evicted and all the court costs/eviction costs etc are added on, you could owe 5 figures (over ten thousand pounds!)
I would rather fork out the first 3 months rent tbh. Which in most cases for uni rooms would be £1250 to £1500 grand for those 3 months, and around £1700 to £1800 for 3 months for many private let homes (in my area anyway.)
I'd rather pay the first 6 months even (maybe £2500 for a university house room,) than be a guarantor.
If you offered the first 3 months, they may not need a guarantor, and if you paid the first 6, they almost definitely won't need one in most cases. Seems like a lot, but at least you know where you stand, and there will be no nasty surprises of ten thousand pounds plus to pay!