The fact that they haven't actually said makes me assume it's not... he would be the most obvious choice, so I see a big surprise reveal that it's *not* him upcoming. The series is only four episodes, right? I wonder if next week he'll be the one being questioned by police (or will appear by the end of the episode in the "now" timeline in some way), leaving the answer to the big question to be shown in the final episode.
Not to mention stealing Marc's Viagra!
Honestly I'm not liking anyone on the show too much, though it's still holding my interest. I think George (the oldest daughter's husband) is the only one I kind of sympathize with (well, the son's wife, too, but we see less of her). (Although I automatically associate the name "George" with Russell Tovey thanks to Being Human, so it's a bit confusing to have him not be George...) All the adult children are pretty awful, as is Mary, and Marc and the mom are being presented in an ambiguous enough way that you can't quite trust them (not that I think the mom is nefarious, but as some have mentioned, she seems way too gullible, and I do wonder if she's hiding something).