It really doesn't matter whether people are fully trans, non-binary, gender-nonconforming, whatever. There is no right and wrong way to present. There is however, an issue about what is considered in good taste. In the UK at least, there is probably a consensus that presentation should be modest, wrt to tucking issues etc.

Ultimately, this is a great situation for you to be forthright in your support of your relatives/in-laws who are part of the LGBT community. This may gently challenge/question the homophobic and transphobic opinions of some of your relatives that you have mentioned in recent posts.
Best wishes.