All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Indeed, accompanying your grandmother, helping her in her ‘Umrah, and attending to her needs is an act of dutifulness towards her. We implore Allah, the Exalted, to reward you for that.
There is nothing wrong with your grandmother performing tawaf and sa‘y in the wheel-chair, but she is obliged to have the intention of tawaf and sa‘y for herself, and you, as well, should hold the intention of performing tawaf and sa‘y for yourself. The fact that you are pushing her wheelchair during tawaf and sa‘y does not affect the validity of your worship because each of you holds his own intention and performs tawaf and sa‘y for himself. Scholars held different opinions regarding a person performing tawaf while carrying someone else because one tawaf cannot count for more than one person. However, this is not the case with you and your grandmother because each of you would be performing his own tawaf and sa‘y individually.
Allah Knows best.