This is it. This is the beginning. Up until this point, there was atyant confusion from every side. They said dealwali shaadi but sometimes both Sai and Virat got carried away by it all, especially Sai. But everything is out in the open now (as open as it can be). Sai caught on to VirKhi being more than devar-bhabhi and best friends in Gadchiroli itself but she was in a "Iss Pyaar Ko Kya Naam Doon" phase. That explains why sometimes she got platonically possessive of Virat sometimes. She thought Pakhi is crossing the threshold of 'friendship' and was overstepping.
Now, she knows how important Pakhi is to Virat. "Mujhe nahin pata tha ki Pakhi Didi itni zaroori hain Virar Sir ke liye." She knows that Pakhi not being around makes Virat sad and restless. This is definitely more than friendship and this is where Sai is going to set boundaries for herself. She won't come in between their god-knows-what-it-is. Her realization of the extent of Virat's feelings for Pakhi came with a tinge of sadness but Sai Joshi is going to convert this sadness into strength. If she was letting herself get swayed by Virat's niceness and their whole pati-patni ka rishta, she won't do it now. In the true sense, she has started building her Sinhagarh ka quila.
As for Virat, I am glad he has realised what a jerk he was to Sai. I absolutely loved when he thought, "Sai mujhe pasand nahin karti lekin bharosa toh karti hai." Good, now please always remember it Vaada Boy. Don't break her bharosa. For the first time, it seems like Virat genuinely thought about Sai's feelings (result ke baarein main pareshani) without it coming from a farz/zimmedari/obligation. This is an indication, he's genuinely growing concerned for Sai but the damage is done. The quila construction work has begun and Virat has to do a lot of work, get the mightiest ghorpads there are to be able to scale these walls. Sai is now clear about where she and Pakhi stand in Virat's life. Virat is confused, as usual but the shift in tide is happening. Everything is as clear as it can be and now is the true beginning of SaiRat.