All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Sharia states that every person is entitled to their right and it is not permissible, no matter what the case may be, to force any of the heirs to relinquish their right except with a cheerful heart and after reaching the age of puberty. Since the young woman mentioned in the above question is fifteen years old, then relinquishing her right to inheritance through her guardian is permissible because according to the Shafie School of Thought fifteen is the age of puberty. In other words, this girl is an adult and her relinquishing her right to inheritance is valid provided that she wasn`t coerced physically or morally since it is unlawful to take someone`s property without their goodwill. And Allah The Almighty Knows Best.