Is it sinful for me to check my brother’s phone to see what he and my sisters are saying about me and my parents in a group chat, considering they are gossiping negatively about us?
Bismihi ta'āla
In this case, it is sinful. If they are all gossiping about you, try to understand why, and seek advice regarding the issues they are raising, to see if they genuinely have a point, and if you should try to look at their perspective. As for spying and violating someone else's privacy, this is haram.
And Allah knows best