Gosh. Wow.
They will go through the grieving process again... and are.
Just offer your shoulder and presence.
Let them vent.
Let them know, you are there for them.
You don't have to say anything... to make them feel better. This is just so overwhelming... and they are all probably "numbed" by it.
Just give your sympathies. And commiserate if they can even do that.
At least, they have gone to Therapy in the past to deal with the Mom's suicide. So they are open to that. That is good.
They will, direct themselves to that again, hopefully, when they are ready.
Then again, some people, just want solitude and space, and privacy with things like this... not having "visitors" yet or having people pop in.
When my Dad died, I was like that. I did not want, well wishers. Even if I knew they meant well. I just wanted to be by myself.
You seem to know your neighbors well... and you have a good sense about the "cues" of others... so use your inner radar. On how to approach them. And yes, everyone needs support in times like this.... whether that is verbal or in person, wishes.
Offer them things/comfort, that you know they need. Or ask for. Or make suggestions. Sometimes at times like this, the person may not even know how to ask for help... or for things that they need. Or they may not even want to "burden" others. But you seem close with them... and so, I am sure they will welcome your efforts or even just your kind presence.
If they like to garden... perhaps get an Olive tree/plant for them. If these are sold in your local garden store. The Olive tree, is a historic symbol of "peace." My MIL, planted one in her garden, when her Husband died. To remember him by. And it is consoling for her. This may be just a silly thing to some, but for others it may be meaningful and a way for closure.