Nora doesn't see the other XJs as Jenny's siblings. They are prototypes. Most of them are designed specifically to test the various subsystems that exist within XJ-9. None of them were meant to be fully functional global defense units with only maybe XJ-8 being the exception.
Jenny is the only one that sees them as her sisters. Nora more than likely does not have the resources to power all 9 XJ units around the house all at once which is why they are in storage. (rather than completely dismantled altogether.)
Also, I don't recall if ever said for certain how much time the other XJ siblings spent above ground before going in storage or even if they had ever even been powered on together at the same time before Jenny found them. They could just have easily been introducing themselves to each other in that episode and not just Jenny.
As for XJ-10, well... you always have to have a contingency. You never know when one day Jenny will go off to Cluster Prime to fight an entire planet and never come back. Also, as with whenever you make something no matter how good it is eventually you see various ways in which it can be improved.