Hi, welcome to the forum
First of all, 4 or 5 months pregnant isn't very demanding yet. The last trimester is.
Since she is almost starved but already has access to pasture all the time (provided there is enough palatable grass), I'd switch from Timothy to alfalfa. Make sure she'll eat it and not share it with the other mare.
You don't say how much mare&foal she gets, that would be helpful.