Just want to clarify that मामी refers to your Aunt on the Maternal side. The names of family members in Hindi depends on how exactly you're related to them.
Mother's brother's wife
Is सुंदर actually used in Hindi? I was told خوبصورت۔ / खूबसूरत is more common in Urdu.
खूबसूरत (ख़ूबसूरत) is an import from Persian. That's why it is more common in Urdu.
सुंदर is inherited from Sanskrit.
It is. Hindi has a lot of words meaning "beautiful", found over 20+ different words (including सुंनदर and खूबसूरत) in just one dictionary.