In Portuguese, that would be a little silly (but not wrong) to say a mãe dela. Someone may say: obviously, she is her mother. In Portuguese, there is no need to add any further information, unless it's not related to the family itself: "ela se parece com a mãe da minha amiga" = "she looks like my friend's mother".
a mãe = "the mother," or in this case probably "her mother." The sentence "She resembles mother" is incorrect in English, anyway; it'd have to be "She resembles Mother."
Plus, in modern usage it's considered extremely formal and kind of creepy to refer to your mother as "Mother." You'd be more likely to call her "Mom."