Awwww......how sad. This makes me feel so badly for you and for your Jack Russell that was close to his feline sister. 18 years is a pretty long time for your kitty to live!
I know I would get to a shelter or a rescue as soon as possible to see the cats that are available for adoption. In fact, if you're not sure how your pooch would react to a new member of your fa,ily, take him with you to the shelter and let him actually SEE the cats & kittens. If he seems happy and excited, as the animals caretakers if you can allow the cat & dog to interact a little. If they take to one another, you have solved your doggy's sadness!
If not, than It will be just a matter of grieving time and he'll be OK.