My corrections are in capitalized letters and my comments are in ( )
1- I go to my ground mother's GRANMOTHER'S house twice a week.
2-I go to the gym on Sundays .
3-How often do you wash your hands? Hourly (I would make this a complete sentence "I wash my hands hourly")
4-I am studying English this year. Do you like to study together? (Good job!)
5-I am visiting two musuniums MUSEUMS today. (Correction as Ana has already written)
6-My sister always study STUDIES well. (Correction as Ana has already written)
7-My sister is always losing her cell phone. (Your sentence is correct. We can use ING verbs to talk about how often things happen. Ana's correction is technically another correct way of saying this)