One of my sister's feet looks like it retained water and never went down, why is this?

Fluid: She could have venous stasis. This can happen if the valves of the veins do a poor job of sending the blood up the leg and back to the heart. Usually starts with swelling, and then a red, itchy rash may develop where the skin is stretching (can be around entire ankle, but usually on the inside mostly). Other possibilities are lymphedema, tenosynovitis, or old injury. See a podiatrist.