Oddly, rinsing the hair with water and fabric softener will also help detangle it. I have often used that (on the advice of a hair dresser) when I had run out of conditioner. Works quite well, and actually had less chemicals than hair conditioner.
You can try unmatting your sister's matted long and thin hair by using comb with moderate width teeth to unravel the hair. Then, use a leave in detangler or conditioner to easily untangle the hair. Consult a professional stylist for more help.