With the twin sisters...I would say that it's a lack of understanding on their part and I wish you the best with your mother talking to them. Honestly, it could be that they've overheard your father saying nasty things behind your back and are just parroting them, as kids are prone to doing.
Personally, I don't know if I'll ever come out to my family and at least part of that is because of my brother, who used to go 'gay bashing' as a hobby when I was a kid. I can only imagine what he'd do to his 'little sister' if I finally told him that I'm his 'little brother' instead.
