Many general European weather sites will provide information relevant to Switzerland. Bear in mind that weather forecasting in mountains is notoriously difficult, and predictions often change radically from one day to the next.
There are plenty of sites showing basic forecasts to help you to estimate the chance of decent flying conditions over the next couple of weeks; I find Meteoblue’s 14 day chart for Fiesch as good as any, taken with their 7 day chart for Eggishorn.
You will probably have your own favourite synoptic charts, but as a Brit, I still favour the UKMO’s. Those at Wetterzentrale and Weatheronline extend much further into the future, but I would only use these pages as starting points for forming a positive or negative opinion of the general prospects which lie ahead, and certainly not for trying to make accurate predictions for specific days more than a week ahead.
The Swiss TV/Radio website provides a simple general overview of the weather situation over the next five days, and a link to the latest TV forecast. In my experience, their general chart of Switzerland tends to predict much more cloud and precipitation than actually occurs, but the forecasts for specific locations, e.g. Eggishorn, showing a projection of conditions every three hours for forthcoming days, are more accurate.
Meteoschweiz has a page showing predicted winds and temperatures at 2000m, 3000m and 4000m for several days ahead. The figures most relevant to Fiesch are likely to lie somewhere between the “Western Switzerland” and “Northeastern Switzerland” headings, as the latter is an inaccurate translation of “Deutschschweiz” (meaning the part of the country where the official language is German). However, I don’t pay much attention to the predictions of wind speeds, as I have often found that the actual strengths which are encountered whilst flying are less than half those forecast here, which I presume is because they relate to the free atmosphere and don’t take account of the protective effects of the mountains. I find this site more useful for confirming a prediction of bad conditions (due to grossly excessive winds or a poor lapse rate) than for picking out good days some time in advance.
The Meteoblue page of winds at various altitudes for five days ahead seems much more reliable, particularly at 750hPa/3000m, which is often helpful to try to decide whether a particular day is likely to be blown out or not.