Heidi Range plans to marry in winter 2012.
She told Britain's Star magazine: "We want to do it in the winter next year. I want to make sure everything's perfect! I've already found the dress, so I just need to find the venue and then everything else will fall into place."
She said: "I've always wanted my sister, my best friend, and Dave's sister."
The original line-up of the band, which also included Mutya Buena, was famous for in-fighting, but although the group are calmer now, Jade admits joining was "very difficult".
She said: "I was the change in the line-up, so indirectly people associated all the change of sound with me.
"There was too much going on at once. I just went through the motions. We were turning up every day just trying to get through it. It was a very difficult time."