We started using family room, but my sister is in another country and I wonder if chat messages are counted as SMS? Or are messages sent over the web?
Room invitations are sent via text message, so your mobile operator may charge you for this service. Once everyone is in the room, you can chat and share using just a data connection. If you have a cellular data connection, how much you pay for the data you use is determined by the data plan you have, which is part of your service agreement with your mobile operator. Connecting to a Wi-Fi network can help reduce your cellular data usage.