Yes, cats do grieve, especially when they've had close relationships. So sorry to hear about your two boys going to RB but I agree with Killer Kitten - adopting another friend for your little girl would probably be the best thing for her. I also agree that maybe an adult neutered male would be best, because that is what she's used to. Getting a kitten may alienate her because kittens are so active and she may not want to tolerate all those kitten antics.

Good luck with whatever you decide, and again, sorry for the loss of your two boys.