My Dog is having trouble putting on weight?

My boxar Indy is not gaining any weight she sooooo skinny my grandma says you can see her hip and rib bones and we dont know why she is so skinny, she is pregnant currently and my grandma thinks she might die if she has them but she is having them tonight, but we dont know what to feed her to help her gain weight we feed alot of food but she only eats a tiny bit, her doughter from her first litter had put on weight, and we have just dewormed them for every worms including tapeworm two weeks ago but i need help what is something good to feed her to help her gain weight? ? PLZZZ help