Multiplayer Problem

I want to play Minecraft Pocket Edition with my sister, who lives with me, but every time I try to join her game, I cannot join, and therefore cannot play with her. I like MCPE because it is relatively easy to play multiplayer on it, but if I can't play multiplayer with my sister, who is using the same Wi-Fi connection a me, I am missing on the best part of the game, playing with others.