Same with my younger sibling. It was very obvios early in their life, but we didn't had words for it either. Didn't understand and didn't took it that serious.
I had my period with 11 years, but suffered till i was 30. Cause it was a tabu to talk about that. I need to Suffer, i was told. BS.
It is crucial to give kids knowledge that they are not wrong. That they are not weird. That they know everything is okay and their parents are here guiding them. AND they need to know that other people are not weird so they can learn empathy.
I knew my gender identity was a lie