How do you get children to start speaking their native language again?

If they do not understand Lebanese Arabic anymore, then that is no longer their native language. A child's native language is the language that a child USES all the time. You can encourage your sisters by only speaking Lebanese Arabic with them and do not understand when they speak to you in English. They probably still understand and can speak more than they are letting on. If they must communicate with you in Lebanese Arabic, they will.