How do I help my brother?

I (22F) have a younger brother (13). We also have a sister (17), who might be relevant later.

Our mother passed when I was 15, meaning my brother would've been about six. After her passing, I lived with my dad, and he lived with his. We weren't very close until this past year or so.

I found out from our sister that he's been acting out and getting into trouble, hanging with the wrong crowds.

And as his big sister, I'm super worried. Of course losing our mom and having a rougher upbringing is taking its toll, I just don't know how to reach out or start that conversation.

Any advice is welcome!