Help

So my sister in law has got a cat and The cat has had babies 3 months ago and she's asked if would it go back to normal after 3 months  also her cat is quite slim is they any tips to help her gain weight as she is thin she only got her today. 

Thank you very much 

Is the kitty spayed yet? She should be spayed soon if not. If she goes outside she's probably pregnant again already :/. So hopefully she's either been spayed or kept inside with no tomcats. Has she been fully vaccinated and de-wormed?

Once she's spayed and de-wormed she should put on weight. You don't want to do anything special to fatten her up; just feed her normally and she'll gain the proper amount of weight over time. Pregnancy and nursing babies sucks the nutrients right out of the poor mama cat so it's pretty normal for mothers to look a bit rough by the time their kittens are weaned.