1.Well, without seeing the content of Will, partition deed and plaint of the suit it is very difficult to comment on the merit of the case.
2.However once things can be said for sure that if the sister who had signed the partition deed and got it registered can not alter retreat and challenge the deed.
3.if she challenges the Will and gets successful then also she would not succeed and the partition deed mutually signed, registered and acted upon can not be set aside.
So relax and let the suit proceed. You loose nothing.