just found out my brother is going to be father again. I should be really happy for him, but I can't seem to be.
You should probably start by talking about your brother or sister
I'll talk about my brother.  My sister doesn't have anything to do with it.
You should be happy for all of your family members when they are excited about something. Just remember that your brother's kid is your relative as well, so whatever excitement you feel, your brother is also feeling.
I was already happy for them, but their announcement just upset me.
You should congratulate them if you are happy about it. Do you feel jealous because you don't have any kids of you own?