Dear . . . . . ,
No; that would be a valid excuse.
I am assuming of course that you have unavoidable contact with your mother (e.g. You live together, which is suggested by your "we") and she has unavoidable contact with your grandmother for the next few weeks (e.g. she takes care of her).
Obviously, skipping Mass should not be done lightly or unnecessarily or as an excuse, but if there is a well-founded concern that can't be worked around, I would not scruple about it.