Amichai, please don’t think badly of your sister – people deal with their lives in different ways and we all cope differently – some by talking some by staying closed up. I’m sure your sister appreciates you in every way, more than you’ll ever know and even more so because you don’t harrass her for information! Carry on caring and try not to be too hard on her.
Joseph you can make a l’chaim between the couple and wait until all family are told before going public. A l’chaim is a committment to the ‘deal’, not necessarily a formal engagement. Some couples do this until their parents or near ones can be personally informed.
Jaymatt19 – please rest assured that once the initial shock was over your mother was as happy as anything possible and all is now forgotten