I played the second cbt and you could choose the second protagonist too, don't think they changed that so you can have 2 male protagonist or 2 female protagonist if you want

They made it so you can now choose the gender of both the mcs :) so you can do double male, double female, or male and female

from what i've read you can choose both Male or both Female too, even if it doesn't make a lot of sense to me lore wise

so isn't that mean if i choose male and i can only see the older sister? although i want to choose male and see the older brother?

yeah