I personally found that my ds's got like this when they were 'ready' for school but not quite old enough- made for a bad few months. I think it's a combination of a new maturity / tiredness / readyness to move on etc and I wouldn't t ake it to heart too much.
I would sit her down and say that Mummy is suprised how much she has grown up lately and how soon it will be when she goes to school. tell her some of the good things about being grown up but then explain that it also menas she has to be kinder to smaller children and behave more nicely. Also give her wind down activities she can do without her sis: age appropriate crafts, a reading corner in her bedrrom (cushions, rug, shelf) or a leap pad type toy. Now's the time to teach her how to spot when it all gets too much and she needs time out.