I have to agree with Paris - isn't there a reason why nobody could possibly get a mortgage on a salary of £9,000pa?

Because any of the things that Paris describes above could go wrong, and there are all those additional costs. At the end of the day, the first tenant your sister gets could complain about the shower, the boiler, the washing machine, the carpets, or other stuff - my dad has been a landlord and says the amount of things that come out of the rental income for whatever reason are such a pain and it's actually quite a challenge to cover every mortgage payment, let alone make a profit. And while you might be fine the first 12 months, the next year could be the one where the boiler inexplicably implodes!
I think a one-off gift or a loan are one thing between family members, but on-going official financial obligations to actual banks with fuzzy 'terms' like "I'll pay it all, you just sign your name on this form, ta!" are quite different. I may sound like a total sociopath here but I actually don't think it's appropriate to put a family member at risk for your own borrowing, so I'm a bit surprised your sister asked.
I looked into getting a mortgage recently but gave up pretty quick because A) you need massive earnings and a huge deposit to get a decent mortgage, it's quite ridiculous these days! and B) the rates didn't look very good and I'm just not in a steady enough position right now to take a risk borrowing more than a tenner. Things can change so easily in life and you have to seriously dedicate yourself to paying off a mortgage if you get one, so you have to know you'll be happy to focus on that for the next 20-30 years, sacrificing all sorts of other stuff.
Sorry - I know that's not much help and I'm sure your sister has done plenty of thinking etc, but I think it's honestly fair to tell her that you're not even 100% about getting through the training period at your job, or that you might want to take a hiatus from work and travel/re-train/whatever in the next year or two, so you're just not in a position to be liable for someone else's mortgage right now.