Has to be Uttara...She seems to be affected the most by Brahma's transgressions and is motivated by extreme hurt and vengeance... She doesn't have the baggage that Devyani or Priya has of losing family reputation etc except for Tara who would be cared for by Jhakad family anyway..
I think they will all have a part to play. Devyani is already exposing Brahma and RD wont think twice about exposing him. Uttara and Priya will most likely testify against him. With the threat of divorce and already supporting Devyani, Priya doesnt have much to lose either.
Gaurav can testify against him too, basically confirming Devyani's story about taking her children and leaving. And also of all that he's seen Brahma do his mother.
(So can Vidya, but I don't think they would want the gay part to come out in court.)
RD & Utara. RD wouldn't think twice when it comes to her only son so thats really a no brainier. Uttara would be the biggest target as she doesn't really have much left to lose. She has managed to embarrass herself enough as it is and this time seems to be genuinely remorseful about her actions. She only has Tara on her side so I assume out of everyone else her motivations, revenge and sudden "care" for her sister would show.