Dependent visa for mother

I came recently to UK on HSMP visa while my sister has been here for close to 6 yrs now and holds PR (ILR) status. My father expired 2 years back, my mother (56 yrs) stays with my only other brother in India. My brother got his HSMP approval as well recently , hence he applied for Entry clearance along with my mother stating that she is a dependent. The EC was refused for both of them stating that my mother has applied as an elderly dependent relative of HSMP applicant, and there is no provision for this in the immigration rules. After this, my brother applied for himself alone and got his EC. We would want our mother to come over aswell as it would not be possible for her to live a life alone there.

1. Under these circumstances what is the best course of action to take. Should we apply for a dependent visa with my sister who has PR status as the sponsor.

2. If so, can we do it immediately or should we wait for some time .

3. Does it strengthen the case if my brother is in UK as he is yet to arrive here.

4. Can we do the application in the UK ?

5. What supporting documents/ evidences should we provide to strengthen the case ?