Allah know better than us … maybe He will accept it…after all, she is not pagan she believe in Allah too…The question is it is haram for me to do so, and if i do it there is punishment for me?
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You must not do Umrah for your grandmother because she is not Muslim. The most you can do is to pray for her that she accepts Islam before she dies. The Prophet salla Allahu alaihi wa sallam was denied from seeking forgiveness to his Uncle who brought him up and protected him from the pagans and defended him with is life and with his sons.