iPads for cello-piano ensemble

My sister and I ( who live 1500 miles apart)are trying to play piano/cello sonatas using FaceTime on our iPads with not much success. I'm the piano player and can hear her cello just fine. The piano does not come through loud enough on her iPad for us to stay together. She has used the speaker on the iPad, earphones, amplified the sound with add-on amplifiers. Do I need to use an external mic?And if so, which one?

For this kind of application an external mic and headphones/earphones, I'd suggest, are a must.

headphones so you hear the mix correctly but also so avoid feedback.

There are many microphones on the market. A reasonably priced first step is the iRig Mic.

Another thing to consider is bandwidth. Have you got a robust enough connection to handle the facetime call? Do you need video? Perhaps an audio only Skype call will suffice?

Thank you. You have given me two options to pursue.