If you keep the downstairs ones partly closed, you should get more circulation to the upstairs one. This, plus continuous running the circulation pump may do it. Its just like with forced air, you partially close the close registers to push hot air to the further registers. Same idea but with water. Also, look into insulating better in the upstairs room. Can you add insulation above ceiling in attic? May wrap plactic around windows to seal and reduce draft from outside? It mat take a combination of things to get the rooms regulated. Worst case, you might have to run a separate pipe to the upstairs room rad and install a separate zone with thermostat for that room. This is actually the proper way to do it! At least get your mother-in-law a sweater for christmas.