Help! Is it possible for tenants to steal internet connections?

[Heather]

My parents are the landlords of the two unit house, where the tenant pays every month. First of all, I know very less about rental and estates, so I may seem to not know about the laws, conditions..etc.

My parents seem to get along with the tenant, because my parents are really nice towards him.

Here what's related to the question. The tenant came up (we live on the upper floor) and asked about our internet modem while I wasn't home. My mom, without judgement said she allowed him to copy the code that was on the modem. She believed that the tenant tricked her by saying that his own internet doesn't work.

My older brother (who cooperates with my parents in taxes, bills, mortgages, wages...etc) and I believe that he is stealing our internet connection.

Does anyone knows if he is actually using/stealing our internet connection? What should I do? My parents only made few agreements such as covering his water/electric bills, A/C+heater, and a parking space and nothing else.