You are going to be aunt to my son.
You're going to be aunt of my son.
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The preposition of when used in familial relationships generally describes genuine relationships: mother of my son, aunt of my son, etc. It is rather formulaic and stilted, however, and not much used. The only time I can recall hearing it is when reading about T. E. Lawrence describing how Bedouins did not refer to women except in relation to a male relative. (I.e., never "my wife" but "the mother of my son.")
To, on the other hand, while performing the same function, also allows the definition of relationship to be broadened. Surrogate relationships can be expressed:
She acted as mother to my son, though she was only a wet nurse.
But the genuine relationship can be expressed as well:
Until her death, she was a wonderful mother to all our children.
Note that to is the more natural preposition to hear in this context, but it is still more common to hear a simple possessive there: John's mother, Sally's aunt, etc.
Addendum: As @Deepak noted in a comment, to is often used in an admonitory tone, as in "You should be a better father to your son!" It can also be hortatory ("Try to be a better father to your son") or aspirational ("I would like to be a better father to my son").
"The aunt of my son" is a woman who just happens to be in that kind of biological relationship with my son. An "aunt to my son" is a person who behaves towards my son as an aunt should behave.
For example, my sister whom I haven't seen for twenty years and who has never seen my son and doesn't even know about him would be the aunt of my son, bcause that's the biological relationship, but she is a total stranger to my son, not an aunt.