There are many reason for a cat not being able to jump or simply not wanting to jump. They can range from leg injuries to hip issues to problems with depth perception to epilepsy and more. It would be a good idea to have your kitty checked by the vet to be safe. I echo BenMax's statement though. If she can't jump properly she really should not be outside - period. Neither should her sister. Better safe than sorry. A dog intent on getting it's prey will do just about anything to get it. I would hate to see your cats come to any harm.
As for a good age to get them fixed - some vets will spay as young as 8 weeks. Some want the cat to be at least 2 pounds. Some want to wait till the cat is six months as before that the organs are very tiny. It will depend on your vet. For right now it is also another reason why it's a good idea to keep them indoors. A female cat can go into heat as early as 4 months.
If you insist they have to be outside and you are worried about them coming to harm with your neighbours dogs (or cars) have you looked into cat proofing your yard so they can't escape? You can do this by either building them a small enclosure that is completely fenced in or you can have a cat proof top installed on your fence to keep them in. Here's a couple of sites that will give you tons of ideas to keep your babies safe. -
And thank you for rescuing them.