I saw my brothers phone number appear on our TV screen yesterday when he called our house, does this mean our phones are being monitored?

You likely have telephone service via your cable TV service provider.  Pushing the callerID info for incoming calls to the set top box is one of the features.   That is not an indication that your phones are or are not being monitored. 

it means

1) Your cable provider also provides your landlines AND

2) Your brothers number is in your contact list

No... 

it means your TV has caller ID. 

This is a common feature of Internet-based phone systems of cable TV companies such as Comcast. Nobody is "monitoring" your phones. You have either been hiding under a rock, or you have a new cable TV service.

You don't really understand the internet or your cable company, do you?