Ragini had lost her mind after Sadhna's witness, things added to it after she lost her child.She was not in a proper frame of mind. To add it all, the master strokes were played by Kaushi and Vasu. Both Kaushi and Vasu knew that Sadhna loves Ragini a lot, what more pain can it be if they somehow can turn Ragini against Sadhna. Actually, what was needed here was a strong understanding from both Ragini and Sadhna. Ragini should have strenly told Kaushi and Vasu that Sadhna was her sister and she would deal with Sadhna in her way.
Regarding Ranveer, my feel is he will first try to take Ragini out of this shock in a loving way and then try to explain things. But after that, if she still holds Sadhna responsible, he will be stern with her. It will show Ranveer as a responsible husband, who is not giving up on his wife, rather he wants to bring back his wife to the right track, in whatever way possible.