My parents are getting a divorce...

Maybe this topic doesn't belong on this app, but I can't post it on Facebook because nobody knows yet. My biological father wasn't really in my life until I was 13. My mom is married to a man that I've called dad since I was 7. They have 4 kids together, ranging in ages 11 years to 7 weeks. Since my baby sister was born 7 weeks ago, my dad has been cheating on my mom with my mom's best friend. I'm very close with my mom and I hate seeing her hurt. And it broke my heart when my 5 year old brother heard our parents arguing and asked me if they were getting a divorce. I mean, what do I even say to that? I just hugged him and tried to be strong. When I'm around there, I'm strong. But when I'm home, I completely fall apart. I'm so angry and hurt. I don't like feeling hatred, but I will admit that I feel some hatred towards my dad. I don't even want to call him dad anymore, but that's what he's always been to me. And now, he won't be there when my son is born. He won't be there to walk me down the isle. I'm falling apart and there's nothing I can do about it. My family is just completely broken. I'm 19, so I'm an adult. Is it okay to still be angry and hurt by a divorce even when you're an adult and should know how to be able to cope better?