Should I be their guarantor?

Fiddlestick

I'd give this one a wide bearth.

mlz1413

I agree with this:
'The fact his own mother has refused to act as guarantor should set alarm bells ringing'

Remember its not just the rent, if they do any damage that will also fall on you if they fail to repair / replace items.

Your sister and her partner have been together 4 years, they have just started looking at flats and have met the first hurdle, let them decide how to get round it.

Help them by giving them questions for the letting agent, ie:
if we can't get a guarantor how do we rent.

They may have to look for somewhere cheaper or put down a larger deposit, neither things should stop them from living together.

mrcol1000

I was guarantor for my brother when he rented his first flat. But he rented it alone and I made sure he could afford it before agreeing. The contract I signed basically said that I was responsible equally for the rent along with my brother so if he didn't pay I would have to. I am not sure how legally binding that is or how it would be enforced. However he didn't let me down and now has a full time job so doesn't need one.
Like others have said though. Your sister is self-employed. Her boyfriend has had issues in the past. Its not very stable. Also if they spilt up would your sister be able to pay the rent herself? If she couldn't you could end up liable and dragged into their spilt.
They are much better of renting through small ads directly with a landlord who won't require one thus saving all the bother.