Minecraft Multiplayer Issue

In my current Minecraft world, everything runs fine. There really aren't any issues when I play by myself. However, when my sister joins my world (multiplayer), that's when the issues start to arise. When she joins, my phone starts to slow down / lag. I ask my sister if this happens to her phone when we start to play, and she says no. When I say my phone starts to slow down, i'm referring to Minecraft only, not my phone in general. This is really weird because it happens all the time. I make sure to boost my phone before we start to play. Yet it always ends up happening. There really isn't any reason for this to happen. I don't use any Texture Packs, even though this happens regardless. My Render Distance is around 6 & 8.
Any Suggestions ?