Hey Nick, with regards to how much you or your mahal have got in your account, honestly I don't have an idea but I think it will be safe if both of you have an account. In my case, since i still have my UK account & a Philippines one, I just show evidence of it, around GBP 1,600 for my Lloyds & Php 50,000 only for my local one. But i don't know if it needs more than that.
For Urs's sister, if she's self-employed, again, it's evidence they want to see that she can accommodate the guest
She might have a business under her name which might help as her asset/income.