If you are planning to travel up the mountains, which I would definitely recommend, you will need some warm clothing. I consider it essential to have something water/wind proof. Several layers of thinner clothing is more versatile than thick jackets.
At the top of Jungfraujoch the snow is permanent. Temperatures are around freezing, even in summer. It may feel warmer if it is a sunny day and not windy.
Even at Brienz Rothorn, there are likely to be small amounts of snow in shady areas.
Look at some of the webcams or weather details on
http://map.search.ch/ http://meteo.search.ch/ to see what things are like before you travel.