Hard to say how much of the volume of non-straight/trans and trans/trans twins on social media is selection bias since the trans/cishet twins aren’t eye-catching

It’s not just selection bias, it’s also survivor bias. Openly trans people are more likely to have openly trans siblings, because openly trans people are more likely to have accepting parents. We don’t know what the actual frequency of trans people is. We only know what the frequency of out trans people is. But the number keeps going up the more acceptance improves.