First, Sakshi had to destroy and break a Mata murti and inside was a key to open the door to get to the first relative/family member.
Second one was Madhura, trapped in a Pavitra Kund. Sakshi had to pollute the waters with her blood and she did so and freed Madhura.
Now, Gauri is hanging above a yagya kund with fire in it and she has to break the Rishi by the yagya kund's tapasya by blowing out the fire and she'll have to face the wrath of the Rishi. This is the 3rd Mahapaap.
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I missed watching Mata Ki Chowki!