The best thing to do is get a vet involved. You will want to check to see if she has worms via stool analysis and if she does, then it's going to be very hard for her to gain.
However worming a very very very thin or unhealthy horse is risky. So it's best to get the vet involved so they can give you advice on what to do to avoid any problems.
Personally I'd only focus on upping the hay (if she gets it all at once, consider putting it in a slow hay feeder) and VERY slowly add in a fortified grain.
A probiotic also is a good thing to talk to your vet about. It helps horses absorb the nutrients from their food, and is imperative to have after giving the horse any kind of antibiotics. AGAIN, TALK with your vet.
The idea to to do things slowly. You don't want to rush the weight on or give them too rich a diet.
16 month colt is 1 year and 4 months old, so he should be weaned by now. Since he's old enough to reproduce, then he's old enough to impregnate his mom (gross I know, but he's a colt) so he should be separated until he is gelded.
How is he doing?
"Strength is the ability to use a muscle without tension"