No offense to your sister, but she's hardly the person to get breastfeeding advice from.
My DD is 9 mo old and still does a substantial amount of nursing at night. It's not comfort nursing, it's EATING. I think six months old is Waaaaaaay too early to start regulating how much they eat.
Keep the following in mind:
-It could be a very difficult, lengthy, and traumatic process to wean a baby so young. do you really want to go through all that?
-It could definitely hurt your supply; do you want to risk your breastfeeding relationship and end up having to supplement her with formula?
-Trust me on this next one: time goes by sooooooo fast. Before you know it your daughter will be nursing at night less and less. I know you are tired, but hang in there mama...this too shall pass. And in the meantime you are doing the very best by your child.
-Finally, your daughter may nightwean all on her own one day, maybe sooner than you think. It's wrong to assume that they "never will". I don't know many teenagers who are still nursing at night, do you?
And you know what? Even if you do get to a point where you want to nightwean, at least your child will be old enough to understand what you are doing, and will be physically able to handle it. I would be so worried that a 6 month old would lose out nutritionally if you start depriving them of feedings so early.
Good luck to you, and way to go for trusting your instincts on this one!