I consciously decided not to tell my mother that I'm dieting, she's had an uneasy relationship with her weight for decades and is a born saboteur. Sometimes the only thing you can do is learn how to distance yourself, practise ways of shutting down the conversation if she tries to raise it, that sort of thing.
By the way, 1200 does sound rather low even though you're sedentary. Try this
calorie calculator. You're much better off eating an amount that makes you feel satisfied and is easier to keep up, especially with extra pressure from your mother to complicate things.
Something I'm wondering about is whether there's a cultural element to all this. I noticed that you're in India. Indian mothers do have a reputation for feeding anyone in sight, and I'm saying that affectionately as someone from a Jewish family - Jewish mothers do exactly the same thing. (I once wondered what Indian Jewish mothers would be like, then realised that I knew a couple of Indian Jewish brothers. They run a restaurant.) There's a hilarious section in a thoroughly mad book called
The Dyke and the Dybbuk where a woman is at a family meal/religious thing, and her aunts are cornering her in the kitchen. As well as criticising her job, sexual orientation, haircut and the like, they simultaneously tell her that she could stand to lose a few pounds *while* practically force-feeding her on a gargantuan scale. I spotted myself showing signs of being a JMIT (Jewish Mother In Training) the other day when my distraught neighbour was sitting at my table talking about leaving her husband. As well as tea and sympathy, I enquired whether she'd eaten that day (she hadn't, and in my experience an empty stomach does not help when you're that upset), and then plonked a couple of satsumas on the table and sort of dropped a few hints until she ate the other one. Respectfully, of course! And she's practically forced food down me whenever I've visited their flat, she's a bit of a diet saboteur herself.