If I leave my job to take care of my father-in-law and mother-in-law, is there anyway to get a supplemental income?

Look into Veterans Administration benefits for F-I-L.

Unless you're wealthy don't leave your job. It can be complicated, and in most cases impossible to get paid for caregiving family members. Look into the in laws finances. They are going to need in home care or facility care soon if not right now.

If they have no funds get them signed up for Medicaid. FIL may need hospice soon which can be administered at home, hospital or nursing home.  Hospice 

If you provide more details of situation there are many folks on this site who can offer good advice from their own experiences.