I thought that the photo was in a frame on a table in the movie? but in the preview, Elias sees it in a photo album, points to the other woman - I assume - and asks Lukas, "Who is this?" I thought the sisters might have been estranged, with recent events having caused them to reconnect - hence a photo showing up that Elias isn't familiar with. Maybe.

Twins run in families;
couldn't Mutter have had a twin, who is now posing as her?
Perhaps to spare Elias from experiencing the guilt he'd feel for having killed both his brother and his mother?
Realistically speaking, no, she couldn't, otherwise why wouldn't Elias mention this? Why wouldn't he mention an aunt that looks exactly like his mother?
Even less realistic is to assume that the mother had a twin sister that the boys never even heard of Besides, the mother took out the pictures of the husband from the photo books IF there was a twin that her sons didn't know of, she certainly wouldn't leave her pictures around like that.
Like you said, it was simply a red herring. People dress up like that all time, I've done the same and witnessed the same thing many times already, it's what friends do, especially if they look alike.
Fine, fine, I'll leave! But first I'm going to bother these peanuts! Hmm? Yes? Hmm? HMM?