Have to see the epi to find out more.
Dadi was angry and didn't want Tanu in MM in the last epi. She said she didn't want Aliya around too but Abhi will ask about her and that's why she's forced to have Aliya in MM. Aliya taunted dadi if she was going to tell Abhi about Tanu's baby truth and the past and hurt Abhi. That shut dadi up. Memory loss has become a convenient excuse to bring the villains back.
If you go by this show's ridiculous "don't hurt Abhi with his past rule," I get dadi's concern for Abhi. I know there's a lot of interest in seeing Abhi and Pragya happy with each other - makes sense. Dadi has a protective instinct as parental figure who raised him. I can understand that. If doctors told me that some people and things need to be kept away from my son for his well being, I will do that at all costs!! If comes down to a choice between health/survival and romance/happiness, I'll go with the first.