I have the same worries as you do. my little sister's struggling mentally right now and she's cutting herself, smoking weed, drinking alcohol, skipping school and she's only 13. She's spoke to me about wanting to go missing before and that she's thought about taking her own life before but she'll only say it to me because she doesn't want our parents to know about it. Just like how you've gone through your entire life with your brother, I've went through pretty much gone through my entire life with my sister and she's been there for me when I've gone through struggles with depression and anxiety.
I pray that both your brother and my sister get the help that they need (my sisters already been to the doctors about it because my parents eventually did find out about it)