.

Remember the visa costs £1600 now and u need to most likely provide and undertaking to maintain her.

These cases are the hardest and will only get harder with the immigration net tightening.

You may have been able to cater for your mother in the past but we're talking long term living costs; not holiday maintenance costs.

when she applied for visit visa how did she convince ECO that she'd go back when her visa expires? what was the reason she gave? she now have to shows the opposite that she has no one back home and no reason for her live there any more.
It is extremely difficult to show that someone is living in 'most exceptional compassinate circumtances'. The immigration guidence are not helpful either. As your mother is under 65 she'll have to fulfil this requirement.
your sister livng there is NOT going to help the case. How can you prove that your sister is in no position to help your mother? Do you think ECO would belive that?

There are lots of people applying for this kind of visa recently. Be expect the settlement visa to be rejected and prepare for an appeal. Appeals can be very time consuming (usually takes just take over an year) and expensive.

Good luck.