It's hard to take a step back and look at this when the situation is so contrived and convoluted....but if you do that, you will see why Adi has no valid cause to believe that Malini could do this right now.
She has painted an effective facade of being oh so caring for Imlie by wanting to go jewelry shopping for her rasam, upar se Imlie also downplayed Malini's nonsense the days prior to this by publicly being all praise for Malini's "support" etc......if Adi had a working brain, he would see the contradictions in why every other day either one sister or the other is accusing the other of sabotaging them......but let's put that aside since working brain is not allowed right now....it's a week to week basis thing.
So last that Adi was aware, Malini supposedly is still all "concerned badi behen" for Imlie, while Imlie is being "immature" or "jealous" or "trying to hurt Malini"....Adi pushed back at all these things relatively mildly only so that can leave room for interpretation that he thinks Imlie is capable of resenting Malini in jealousy and doing all this. He seems to have conveniently forgotten what all IMLIE did for Malini for so long.....lol just cuz Malini is the one who keeps bleating non stop ki maine yeh ehsaan kiye, woh ehsaan kiye blah blah.
And now to make matters worse, SK shot Adi...so now Adi's mind is diverted in the anger over that and it's primed to believe that Imlie is insecure about Malini, so she is mistrusting her and blaming her for SK's actions.
Malini's goal is to basically paint Imlie as an immature, insecure person who isn't worthy of being Adi's wife.....so no matter how many times Imlie nearly dies to save Adi's life, sacrifices everything for him, for Malini, supposedly she is still too "immature" overall so Adi will keep on believing she could be childish to Malini.....lol for this we are expected to pretend like the first 150 epis of story never happened. Sure sure😆
To make this contrivance happen, they have essentially made Adi forget the first 200 episodes of Imlie's sacrifices in always thinking of Malini first and wanting to walk away so many times.....now suddenly just cuz Malini keeps screaming about her "tyaag", Adi has forgotten the actual and original tyaags of Imlie.