If I could ask a question of anyone, I don't think I would ask either DB or JI as I wouldn't likely get an honest answer.
I would ask the neighbor on either side of the Irwin home, and a couple of neighbors to either side and across the street, what they can tell me of the family in the Irwin home.
Neighbors usually observe and have an opinion about the people who make up their neighborhood. If you talk to the neighbors, you'll get a pretty good idea of what's going on with one particular family in the neighborhood. Some neighbors have interacted with the family in either a positive or negative way. If you pull all that information together you'll probably have a clearer idea of character.
Of course, of the neighbor woman who was reportedly drinking with DB that day, I'd ask specifically, "did you see baby Lisa that day and at what time. Did she seem sick and what did you observe? Did you see anything that concerned you?"