Sis is afraid of having double the competition.
She ain't right, but I understand her fear. If he were to have eyes for another woman, she could do whatever is in her feminine powers to out do that chick, weather it was weight loss, better style, make-up, more tender, put down the punani, etc.
But if her husband develops eyes for another man, what can she possibly do? Grow a penis? Be more masculine? Would she even know how? I think that's her and many other straight women's fear. Being pushed out of the loop and having no control.
If you didn't understand bisexuality, I think anyone would have that fear. I mean, many gays do anyway even if they should understand alternative sexuality. "What if he falls for a girl? There's nothing I can do to compete."
I think it's a perceived lack of control thing.