Lets look at this way. Ragini & Sadhana character is someone's creation and only that person knows why they act certain ways.
There are reasons why I think its very unlikely that Sadi will be responsible. This is based on whatever is shown so far.
1. Kaushi made Sadi life hell by constantly reminding Sadi through her actions that she is a bad omen for Ragini. Sadi being so nice was always under pressure to prove herself. Whats best about Sadi is that she never felt bad and understood Kaushi's feelings. Even in yesterday's episode she was doing Mamiji..Mamiji but Kaushi didn't even bother to look at her.
2. Sadi knows what happened between Ambi-Vasu. She will go to any length to make sure this does not happen between her and Ragini for simple reasons. She is the elder bahu and has fair complexion. She already thinks its her responsibility and she needs to be extra careful. Remember Kaushi said the same thing to her during Ragini's wedding.
Please don't bash me for saying this. This is how I have interpreted the story so far and I am not taking anyone's side.
I agree with u Anu, and by the way things are going it seems that Ragini will be DBua's fav. and she may shun Sadi away, so more chance of Ragini being influenced by DBua than Sadi.