... but here I want to add that u can pray namaz as qaza for ur parents too... I'll try to find the reference as I read that in a book written by Imam Malik...
And if u know that ur Parents made a promise to someone or took money from some one or took responsbilty of any matter... u can fulfil it....
May Allah give them place in Jannat ul Firdaus and have mercy on them...ameen
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sister, i believe u can NOT do qazaa prayers for a deceased loved one coz its famous phrase in urdu that after death ur ibaadaat come to a halt while ur good deeds [like sadaqa-e-jaariya (continual deeds)] continue to accumulate. plz find references coz i'll be very surprised.
yes sis, u can fulfill his/her obligations including marriages of ur sisters/brothers who haven't been married yet. Allah ham sab ko taufeeq 'ataa farmaaye k ham apne vaalidain ke nek aulaad baneN unkii zindagiyoN men bhii aur unke visaal ke ba'd bhii. shukria**