Please don't think you are betraying them. They have a special place in your heart that will never be replaced by another cat but there is room for many there. Something pulled you to the SPCA so I think that means that your heart and mind are ready.

I think it would be best if you could be there with him while he adjusts especially if he is special needs. You may need some time to see what kind of adjustments you need to make in your house for him. He probably won't need any special things but I'd hate to find out the hard way. I would talk to the shelter, explain the situation and see if they would be willing to hold him for you (maybe you can go ahead with the adoption and payment).

On the other hand, if the shelter won't hold him and you know your sister will be taking good care of him then maybe you should go ahead. Not much help, am I?