Wǒ mā hé tā cóngxiǎo shì hǎo péngyou, suǒyǐ tā chéng le wǒ gānmā.
My mom and her were best friends since childhood, so she became my godmother.
Tips
culture
干 here means "dry" but in kinship marks a relationship without blood ties. 干爸/干妈/干儿子/干女儿 are common informal honorary bonds sealed over a meal, not legal adoption.
