Where there is this only son-no husband scenario, this kind of a reaction isnt uncommon. Again, another issue that exists in modern societies that our makers might subtly tackle.
Its sickening. The mother isnt expecting her son to get physical with her. No... The mother however is used to being the center of her son's universe. She will share him with his sisters, his work etc. Things that have been around/grown around him as he has grown. She will share him with his friends. She will share him with gym equipment. But the moment there is another woman at the ripe jawan age of 28, this mother gets insecure. This has nothing to do with sex. But she knows, this new woman is who her son wants to spend his life with. She knows her son has needs that only this new woman can meet. And that's when she starts acting weird. Especially when there are no other males of her son's generation who are married. This insecurity goes up when she knows this new woman is a lovely person and a better person than herself. At some level, Ishwari is possibly going through these thoughts. She has seen her son in immense pain when this woman had to temporarily leave. She knows this new lovely woman is what her son needs. However, all along she was the one who met his needs and starts feeling insecure. She knows the sex is inevitable. She knows its healthy. She knows its going to happen. But she still feels uncomfortable.
This is a real test for Dev.
Having said that, I dont think Ishwari was trying to stop the consummation. She was uncomfortable at the thought of it (just like a dad might feel for his son in law at some level).