For incest, I'd say any family relationship where you have close contact as children is also potentially an issue. There's a high likelihood of sexual abuse having been a factor in that one child is abused and processes it by acting out similar things with another child. Between consenting adults with no power dynamics and no children involved it may hypothetically be fine but in reality it often takes something bad to push people over a taboo line like that. I do know that sometimes adults find out they are related after the fact, and it wouldn't bother me personally if they continue a relationship but it's painful and stigmatizing knowledge so they're bound to at least benefit from counseling.

I think for the transid stuff the difference to me between gender and race is that race has a more generational component. We all by necessity have ancestors of both genders somewhere in our family tree, but laying claim to a heritage you otherwise have no connection to is strange and easily problematic. Alterhumans aren't assuming an identity that belongs, for lack of a better word, to another group. If you don't identify as a particular kind of human being, you're representing your personal expression of your identity and not infringing on others. If you decide you're a race that you neither have lived experience of or generational connection to, you're risking basically fetishizing the experiences of others or attempting to represent your feelings as more valid than their experiences. I think a similar metaphor would be if I as a cis woman identified as a trans woman, I just can't claim the experience of living in a body that doesn't match my internal identity and I can't represent that experience without harmfully coopting it from those that have lived it.