A lot, yes. We've already seen a few exceptions like mother/father, sister/brother, daughter/son, and there may be some exceptions among professions as well. But yes, in general we can make a noun or adjective feminine by adding a ta-marbuta (ة) — which handily is very similar to adding -a in many Romance (and other European) languages.
Certainly it is ambiguous and it can mean "My grandmother (is) Samia". If you would like to say and emphasize "My grandmother is Samia", then you could say جدتي هي سامية (lit. "My grandmother she Samia"), since the third personal pronouns could take a meaning similar to "he is, she is, they are", etc.