@gabclement - you were communicating through iMessage before. If you turn your Chat Features on your android you could get around this, but then you would need your data on when not in wifi areas...so, I would not recommend this.
Best to get your mom's settings changed (as @dust2dust mentions).
Ask her (or help her) perform the following steps to try and fix the issue:
- Connect to the internet using WiFi or Data.
- Go to Settings > Messages and toggle iMessage OFF.
- Turn your iPhone off completely, then turn it back on.
- Go back into Settings > Messages and toggle iMessage ON.
- Make sure “Send as SMS” and "MMS" is also turned ON.