My MIL ( four of them as i m married into a joint family) insists that for the first couple of months following delivery ghee is the food of choice ...it's becoming very difficult to convince...will it cos weight gain or will the calories be used up in BF.?

Why would you want to eat ghee ? It's not going to make more milk or make you stronger. Whatever normal diet you are following from the third day is what you need to follow. The concept is healthy balanced meals.

Yes you can have ghee. You will start on liquid diet on the next day and soft diet on day 3 by evening. You can have ghee with soft diet. Avoid with liquids