Susan, oh my! That is an awful lot of loss in a short period of time. It is very hard when they should still have a lot of life left in them.

How many of the cats were related? It sounds like maybe there's a genetic heart issue going on--certainly with the three siblings you've described. If your other cats potentially have either of the same parents, maybe there's a problem there.

My thoughts and prayers are with you in a very difficult time. You're right, they are all special.