Your daughters must communicate Ohmother?
So why is your daughter not speaking to her Sister herself?
She could, as others have mentioned, ask her, if she was having bridesmaids or not? Then carry on from there.
I think that it could cause problems, if you "nudge", as you say, as she may think that you've both been talking behind her back.
My daughter's best friend was getting married, her twin girls 7 asked would they be asked to be bridesmaids, as it was "Auntie* ----
When she realized that it wasn't to be, she bought them both the most beautiful party dresses (as near as bridesmaids, without being obvious) and they had a great time, without all the stress and nerves of walking down the aisle etc.