If she were my mom, she would have had me when she was 10.
If she were my mom, it would mean she had me when she was 10 years old.
For her to be my mom, she would have had me when she was 10.
英訳1:仮定法過去で話すので、主節は would have ~ の完了形になります。
have(人)には「(子供を)授かる/産む」という意味があります。
英訳2:it would mean ~ で「〜ということになる」という意味。
英訳3:for her to be my mom の直訳は「彼女にとって私の母親になるには」ですが、「彼女が私の母親だとしたら」という仮定のニュアンスを含みます。