Hi Sonal2007, and welcome to MySwissAlps!
Well, you have access to a better weather forecast then me (who lives in Switzerland), because mine tells me only about the next five days. Let me know, I'm interested...
More seriously, first you could be optimistic as the weather on 4-6 June might indeed be different than what you think now. The total number of annual rainy days in Switzerland is rather small. Well, when I say rainy, that includes snowy if we are talking of Jungfraujoch which is above 3000 m elevation. Few rainy/snowy days, but more cloudy days, sure.
Whether you would enjoy Jungfraujoch depends mainly on your tastes and preference, and the spirit of your group. Look at it this way: it is reasonable to stay on Jungfraujoch for two or three hours before taking the train back to the valley. People usually go to the outside terrace for 10 to 15 minutes, to enjoy the view and take photos, after that they feel cold, even on a sunny day (it can be windy), and tired (they just climbed 2000 meters or more, breathing is difficult). They spend the rest of the time at one of the restaurants, and visit the ice cave and other features, I think you can even make your own chocolate up there. What we Swiss do on a cold and rainy day is eat cheese fondue. You can do that (probably, I haven't seen the menu card) at one of the restaurants, or you can make it yourself! Or keep that for Zurich.
Instead of going to the top station of the Jungfrau Railway, you could stop at Eigergletscher and enjoy their world famous pastries...
I have never been to Titlis but I assume you can go there on a rainy day too. At least the crowd will be thinner. The same holds for the Rhine Falls.
To advise you on other rain weather ideas we 'd need to know about your tastes. There are fine museums everywhere. There is the Technorama in Winterthur, or the Transport Museum in Lucerne.
When I eavesdrop other tourists' conversations, I often observe that they discuss matters of back home (maybe I do that myself when I travel...), rather than sharing the feelings they experience when travelling. If that is also your case, then weather is not so important, is it?
Good luck, and be singing in the rain!