Yes, this is the hardest time. Diagnosis, acceptance, shifting gears..... Hopefully, once you connect to the local Deaf community, you'll be supported in the knowledge that you (and your ds) are not alone in this.
What state are you in now? I can see if I can find a email list for your area for Deaf/HoH events, announcements, workshops, classes, etc. There is often one in each state. If you're still in Wisconsin, I found this group- Deaf/HoH Outreach in WI- outreach
But definitely call the school district, or the early intervention program in your area. Once you get in the system, you'll meet teachers, other families, etc. and connect to the local Deaf community. Some communities have monthly gatherings at pizza parlors, coffeeshops... It all depends on where you are and how vital the Deaf community is. Usually more vital around Deaf schools, but maybe you're in a big city? It might be worth it for you guys to visit WSD sometimes, maybe try to find out their event schedule and go for a play, or an athletic event (sports are HUGE in the Deaf world) just to see what it is like. Visit the preschool, again, just to see other Deaf kids and adults, talk to other parents. Also good for your dd to become aware of her brother's deafness. Often Deaf schools host ASL classes and workshops for parents, sometimes they host or know of 'silent camps' where families go for a 'silent weekend' to play ASL games, tell stories, learn, and just develop community. Again, it all depends on how vital the community is.
Strength and openness to you, mama.