This is a bit of a puzzle - do stop me if you think I'm mistaken.
I previously said that I thought the twins were shot first, but I'm rethinking that.
The magazine held 10 bullets. We know that Nevill was shot four times upstairs, but not fatally, and we know that June was shot seven times. However, IMO June was shot five times at first, and the other two bullets came later. I think that because two of the bullets would have been pretty fatal, and we know she managed to move around the bedroom for a while.
So that's nine bullets. What happened to the tenth one?