Okay first of all I don't understand quite why your parents wuould buy you two pregnant mares if you have never owned a horse before and ones with physical problems at that, but its none of my business. However if I were you:
I would have a vet check out both mares that are pregnant. They should be getting their shots every other month to help prevent them aborting the baby. Specially if your not sure when they are due. Have them vet checked and let them give you a guideline on what you should be doing. Owning two pregnant mares is a big thing and those unborn babies NEED to have special care as well as the mares. When it gets closer to them birthing you will be able to see a few physical signs such as the belly dropping, teats will swell up and wax over also, but I can't stress it enough to have them vet CHECK!
Also if one mare has arthirtis you SHOULD have her checked for the extent of it, before you even think about riding her, again your vet MUST check and make sure certain meds are safe for her unborn before you give it to her anything.
Also if your other mare has a leg problem, that could anything from mild to extreme. I hate to sound like a broken record but this mare needs to see a vet to determine what the problem is and how to treat her.
You have two pregnant mares that both have physical problems, and no one can give you the rememdy because we can't see them and watch them. Its quite apparent they need some care and some help. But we can only read about them, so for the safety and future of your two mares and unborn babies, HAVE A VET TAKE CARE OF THEM......