Mr Jatinder i don't agree with this point of u'rs "their is only one little problem which is online payment does not shown in FORM 26 AS"
Generally in CA Firms, there will be online account & most of the clients tax payments are made through it. It is not compulsory that every one should have a specific online account for themselves to make their tax payment.
But yes care has to be taken with regards to PAN & AY.
The taxes paid doesn't immediately reflect in FORM 26AS, it may take 2-3 working days.
So Mr Anshul, You can pay the self asst tax for your sister through your online account, no problem at all. just be careful about the PAN & AY.