Wow - I admire you for taking care of such a large family, especially when you're so young! And if shaving is what brings you to this site (rather than "blended family" probs, etc) - then you must have things really flowing along. :-)
You asked only for razor suggestions, but here's an unpopular thought for others to consider, : Does she have to start shaving at 11? Maybe she's getting teased about having hair or there's an odor problem, but if not, I just wouldn't worry about it unless she does, if you know what I mean. Our girls in particular grow up all too fast, imo.
Your 9 yr old will learn that she can't do everything her older sister does, and eventually, she won't want to, as their individual personalities will become more apparent. Could be time for both of them for a talk about puberty and what to expect, emphasizing the fact that it doesn't occur at the same time even in sisters? And maybe with the start of shaving could come the start of new "grown up" responsibilities? Not sure if anyone has tried that or if it would be beneficial to your family.....just throwing it out there.
Good luck to you and your beautiful family!