Statistically, there is a non zero chance that 4 siblings are gay. Like if you launch a coin 100 times, it could give 100 times tail.
Only case I can think of off the bat, of two siblings being both lesbians are Tegan and Sara, but I'm sure there are more.
Like I said before (I don't remeber where, uops) I'm lesbian and my brother is gay, one of my best friends is bisexual, like her sister and I know other two pair of sibling who are both bisexuals and one bisexual and, the other, gay. I think that, at least here, it's pretty common xD