Getting a
residence visa for an Adult Dependent Relative (ADR) is very, very hard, even for British citizens. The success rate is only 6% of all applications before appeal and only 22% after appeal.
You will need to prove (with documentary evidence) that
a) Your mother needs assistance with basic daily functions (washing and cleaning herself, etc)
AND
b) Such care is either unavailable or unaffordable in her home country.
The latter is much harder to prove in countries of the sub-continent than in countries like the US, where care costs are exorbitant.
As you have mentioned already, if she is refused an ADR visa, she will likely be refused all visit visas in the future, because of having shown an intention to settle in the UK.
Topic moved to the Family Member Immigration forum.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.