"She's let the kids believe that he is me," John shared on Reddit.
"We're identical twins with the small exception that he had facial hair and I do not, BUT other than that we're pretty much carbon copies."
"I know I'm to blame on that part"
John first noticed something wasn't quite right when he started regularly babysitting the girls after their dad's death in August.
The first time they called him 'Dad', he corrected them and said that their dad wasn't here.
"They asked why and I wasn't sure what to make of it," he said.
"I wasn't sure if my SIL was going to tell them at a more appropriate time so I just moved on.
"I know I'm to blame on that part."
"It's just cruel"
It quickly became apparent that John's SIL wasn't going to tell the kids directly that their dad was gone.
Instead, she seems to be happily living the lie instead of letting them know the truth and allowing them to grieve.
John said he feels incredibly uncomfortable with the fact that his SIL is forcing him into a position where he is lying to her kids.
"I think it's cruel on her part to do that to them," he said.
"I'm also concerned that at a certain point, I can't 'reintroduce' myself back into their lives because I look exactly like him."
"This is how she's coping"
John is also increasingly concerned that his SIL isn't just doing this for her kids.
Although she has never said anything to John or made any advances – he gets the feeling that she also wants him to be his twin.
"In her mind, this is how she's coping," he said.
"By pretending he isn't really gone because she 'technically' does still see 'his' face every day.
"Would I be the asshole if I told them that their dad died and it's really just uncle J helping around the house?"
"Don't f**k them up further"
The commenters were unusually united on this one – telling John that he needs to put a stop to this as soon as possible or risk damaging the kids even further
As one commenter eloquently put it, "This whole scenario is so messed up that I don't really know where to start ... but its s**ty of you to let them think you're their dad".
"This is so incredibly effed up. I'm sorry about your brother, but this is probably the cruelest, most unhealthy way to address the situation with the children," another added.
"You need to put a stop to this now. Don't f**k them up further," a man said.
"Shut this down immediately"
Many commenters were baffled that John had allowed the charade to go on as long as it has.
They were confused why he didn't put a stop to the kids calling him dad straight away.
"I'm not trying to be a dick, and I feel truly sorry for you, but I'm just trying to figure out how this has escalated so far," one man said.
"The whole situation is beyond weird but it does sound like this is how your SIL is 'coping' with the loss," another added.
"Tell them TODAY that you're not their father, that he died, and that it's OK to be sad that they won't see him anymore."
"You stop going over to help, you stop going round to babysit, you speak to her about it and you shut down the 'they call me daddy' s**t immediately," a woman said.
"I knew what happened even if it felt like dream"
But the most powerful comment came from a man who lost his dad when he was a kid.
The man said that his mum and other adults in his life were extremely upfront about the fact his dad was gone.
He remembered attending the funeral even though as a young child he didn't completely grasp what was going on.
"But I knew what happened even if it felt like dream," he said.
"It wasn't his death or death in general that made a grandiose impact later it was grownups and their drama, their emotions and behaviour about it that influenced and shaped mine after a while.
"So people, don't shelter kids from death - they understand more than we think. And they deal with this better than grownups."
*Name has been changed
