You misunderstood, heh. It's not uncommon for people in the trans community to refer to other members as "siblings" or "brothers and sisters" regardless of actual blood relationship.

That said, my sister is non-binary and I'm pretty sure my mom is heavily closeted based on a number of things she's said to me after I came out.

This isn't about people who are trans, but there is a fascinating thing called the Fraternal Birth Order, where every male child a mother has, their likelihood of being gay goes up significantly.

Twenty years ago, Ray Blanchard and Anthony Bogaert demonstrated that the probability of a boy growing up to be gay increases for each older brother born to the same mother, the so-called fraternal birth order (FBO) effect. Their first investigation indicated that each older brother increased the probability of being gay by about 33% (1). This startling phenomenon was confirmed in multiple studies based on independent populations totaling over 10,000 subjects, and a meta-analysis indicated that between 15% and 29% of gay men owe their sexual orientation to this effect