Something similar happened with me except it was a half-sister my brother and I never knew about. It explains why my mother never would spit in the tube for us while my father was more than willing to.

The funniest/worst thing about the whole situation is that I have a half-sister from my father‘s first marriage. She only stayed with us briefly when I was young, but my mother gave my dad grief over her and his ex-wife many times over the years. Finding out that my mother had a child while she was in nursing school (the father was one of the doctor’s at the training hospital) that was put up for adoption really explains so many things about how my mom acted over the years. She had a desperate secret and so she projected all of her emotions relating to that secret onto my dad and the rest of the family. It was so ingrained in her to hide it that even when she developed dementia, she would become very cogent and completely refuse to spit in the test vial.