So perhaps he meant that he didn't know that they were dead at the time they just looked to be asleep?
So if that's his story then presumably he'll say that the 2 of them go to bed and at some point she injects him and then herself.
Who started the fire who knows
What's in the injections unknown
Why did Kate call the mother in law?
Actually do we know it was Kate who made the call? I don't remember now.