Sugu,
God bless you sir, for being so lucky, to have a step mom who was par with your own mom re affection and feelings. She was a great lady, and my heart goes out to you, as you wish to do her last rites.
re the theology angle of it, i do not know.
i would like to comment on one aspect of the human angle, more from life experience. so please treat this as just one view, and perhaps not a correct one either.
you mentioned the feeling that one of your siblings demurred re your doing the last rites. i would heed to that opinion more than anyone else's.
death is a strange phenomenon for those who are left behind.
most of the time we are so shocked and aggrieved. there is neither time nor occassion to think logically, as emotions sway all our actions.
in this context, perhaps your other siblings felt a sense of solidarity or respect towards you and accede to your request to do your sithi's last rites?
again, motherhood is even more emotional. ties of a blood mother, no matter what, have this hairline priority over even the strongestmost other bonds bar one.
i think, sugu, the onus is on you, to further close the hairline and cement your bonds with your siblings. believe me, you will not regret this.
this sibling who disagreed, was perhaps trying to enforce that bond with his biological mother. whereas the others might have been awed of you to talk out their minds?
the others, maybe, are too engulfed in their grief to dissent?
what is your best interests? is it not the sibling love and affection to be maintained. and grew, now that there is no mother to smoothen out any differences?
what if you step aside, and let your next male sibling do the rites?
would it not raise their respect and regard among your brothers and sisters? would it not deepen their affection for you.
i don't think any one of them would feel any less love. only more. please think of LONG TERM...
also, it shuts out any small talk, that may be going on around you that you may be unaware.
ultimately, i think, there are enough poison minds, to start a smoke of doubt and jealousy, with the ever hanging theological sword of damocles always hanging over your head re your proprietariness to this primary role. why give room for such?
in the context of long term bonding and further strengthening your family ties, i think, it is best you let someone perform. be there fully in body and mind, in a supportive role, throughout the functions. you are establishing leadership post motherdom.
you would not have offended anyone, or give cause for any future misunderstanding. the whole family wins. long term too.
to, it is the right thing to do.
i do beg your forgiveness, should any of the above words transgressed your feelings, as i am quite aware of your sorrow and loss.
i wish to reiterate, this is just one view, based on your very brief note. it does not have any intention to hurt anyone.
God Bless.