Is it possible to issue guardianship over a disabled grandmother of the 1st group, if there is another...?

Yes, you can.

Any relative who wishes to issue guardianship and has legal capacity can be a guardian.

I would like to inform you that if you are a man and wish to receive a postponement of the draft in this way, the TCC and SP may have questions for you (there is someone to take care of your grandmother besides you).