Ashwini was the first supporter of Sai in CN. Lot of times she went against her family, against her husband and even her own son too while supporting Sai. Sai gave Ashwini the courage to raise her voice against injustice done to her by her family. She was also the reason Ashwini’s years old marital issues also were resolved. That's why I feel Ashwini loved Sai not just because Virat loved her but she indeed treated Sai as her own daughter and loved her. Lot of times Ashwini trusted Sai more than her own son. The day Sai left the house that trust broke. That day not just a mother-in-law's but even a mother's heart broke. That day she lost her daughter. Then she saw her son also in depression and was on the verge of losing her son too.
After Virat married Pakhi, Ashwini requesting Virat to accept Pakhi was more of a compromise but she started valuing Pakhi only after coming to know that Sai is alive. As a mother she again felt betrayed that her daughter didn't come back to her, Virat's wife didn't come back to him. That's when she started comparing Pakhi's love with Sai's love and thought Pakhi's love for Virat was more truer than Sai's love and the reason was Pakhi never left Virat's side. She feels Pakhi loved Virat even when he was Sai's husband and was madly in love with her. Remember she called Pakhi a jogan? She thinks Pakhi's love for Virat is unconditional are more truer than Sai's because Pakhi will never leave Virat just like Sai left him. That's why she's supporting Pakhi and as you said, she's not gonna accept Sai that easily because she'll always have that fear and insecurities that Sai will again leave everyone and again she'll lose her daughter and Virat his wife. No matter how much Ashwini supports Pakhi I would say that its pretty much evident Ashwini treats Pakhi only as her daughter-in-law, not her daughter. That beti ka darja that she gave to Sai, she hasn't given that to Pakhi. She's supporting her only as an MIL not as a mother.
Ashwini too will flip but only when her bhram that Pakhi will never hurt Virat will break. Till now she's thinking Pakhi is a victim, a poor wife taken for granted by her husband. The moment that bhram breaks Ashwini will stop supporting Pakhi.
3