i am sorry to hear this, I have had a cat with it too, but my other cat didn't go on to develop it, as Angie says, it isn't contagious in a normal way - they will shed the corona virus which your sis's other cats will pick up, but it depends on their immune system, if it is very good, they will shed it completely, if it is fairly good they will shed it initially but could go on to develop it later in life, if it is poor it will develop into dry FIP, and if it is very poor iw till develop into wet FIP, which is what causes the straw coloured fluid. Unfortunately wet FIP tends to act very quickly, it is up to your sister whether she thinks Seb has a good quality of life at the moment, some people have used Interferon, but I have only heard of it being successful with dry FIP, although someone on here has just started it with a cat with what sounds like wet FIP.