Why would "théann mo mhathair" not be acceptable? Not sure I understand.
Téann comes from the verb téigh which means to go (ie from 'here' to 'there'/ another location)
This Q asked for the verb to come (from 'there' to 'here')
The irish for come is - tar, the present tense of which is tagann
Ni theann mo mhathair would mean my mother does not go.