How do you book airline flights to connect with another passenger? What is the best/cheapest place to do it?

I am flying across the USA next summer from coast to coast to visit a friend. My little sister wants to meet me in Texas to fly up to Seattle with me. Her other option is to fly as an unaccompanied minor which would be less than ideal. However I can't figure out how to plan two flights so that we would be on the same plane and preferably sitting next to each other. Does anyone know a website that i could book the necessary flights and still find good deals on prices?