My sister and I cannot connect to each other's worlds.

My sister and I want to play Minecraft together but neither of us can connect to the other's world. We both have XBOX Live accounts and are on iPad. We can both connect to my cousin's world (who is on Windows and in another country.) My sister and I are connected to the same Wi-Fi and have each other added as friends but I can't go to her world or her to mine. We can invite each other and see each other's worlds under "Friends" but when we tap on it it loads for a couple of minutes before the dirt background appears with the "Unable to connect to world" message. We tried restarting the device, and Wi-Fi router, and tried over the course of four days but nothing worked.