I'm so sorry you're in this tough situation. I know at least in my own experience, finding this out about my parent, made me feel like I had caused it. I hope you aren't feeling that way because it is most certainly not your fault.

My TL;DR background: I'm 21F now, but when I was 7, as a result a terrible marriage and divorce, my mom was flung into a severely depressed state and attempted suicide. Since then, I've always maintained a strong and very open friendship with my mom rather than the traditional parent-daughter. My mom tends to be somewhat pessimistic, still struggles with anxiety and takes antidepressants. I often initiate discussion about her stressful job, her general stress levels, her diet, mindset etc. My little sister (I'm one of two kids too) causes my mom a lot of stress and I try to give her advice on how to deal/mother my sister sometimes too. In general, I try to make my mom not worry as much about the terrible shit of the day and make her laugh, find enjoyment in little things, be silly, encourage her to pursue her hobbies by showing my own personal interest in them. (She really into needlepointing so pretending to be interested can be a little tough sometimes...)

I don't know about your own relationship with your dad (it seems yall are very close) and I'm sure that this is very hard and stressful to deal with. If you can, to the extent you can (because you are most likely going through your own stresses-- I didn't like being 17 very much), make it an exercise to talk to your dad about his day, job, interesting stuff he did, even your brother-- try (if you can) to help him talk it out. Talking out every day stresses can really help with everyday anxieties for people which helps in alleviating some long term anxiety. It might not be deeper conversations initially (about sadness or even the things he wrote about in his journal) but I think eventually, as you grow and you and your dad's relationship grows into that type of relationship, it will help him and even help you feel a lot better. Also, encourage him to pursue his art whenever possible! Maybe, to start- ask him about that cat and dog cartoon you were looking for and if you could have it or one like it!