My neighbor cheated with his partner's sister, and now lives with the sister. The most aggravating part of it is that he has kids with both. Every other week his ex has to come by to drop off her kids at her sister's house since they share custody. From the kids perspective, their sibling is also their cousin.

Her ex's new girlfriend.

Idk probably her mom. They're super close, and her mom is in the later stages of life.

That'd fuck things up real bad.

Your partners child.

Hitler, probably. I feel like that might be quite bad.

The one that we made a sexy time video years ago that I have since forgotten that it was still in on my computer, that she found and it caused a big argument.

Magic Johnson

When both people in a relationship are in the leadership of an organization, it's a serious problem when one of them cheats with someone else also in the leadership of that organization. A friend of mine was part of an organization where this happened, the drama was terrible.

My husband’s ex-wife cheated on him with his brother and best friend. It would be completely terrible to cheat with those people even without that history, but it makes it 10x worse.

Stalin

Her Grandfather.

Her sister. They're close, and it would be devastating to her.

Someone else that they trust a lot that they introduced you to to help you out of a jam.

I've seen the best friends one a couple of times and it completely destroys the dynamic obviously. One guy isolated himself and spent a year drunk and on /pol/ and the first conversation I had with him after that was about how the holocaust didn't happen. Idek if cheating happened in that instance or if she just lined up his best friend really quickly.

Everyone new I meet seems to be polyamorous now but I just hear about less relationship drama generally.

Piers Morgan. Sucks to be cheated on, but imagine he was the one your partner preferred. That shit would stay with you for life.

I'm torn between partner's family member, partner's best friend, & partner's ex.

The answer is…. D) all of the above/ and below!

Their sibling, or parent.

Their ex after me, I would suppose. That'd sting.

The Pope

Kim Dot Com

Anyone, but also 2x anyone with an incurable std.

There's only one right answer to this question and it's the one person that will make him/her feel stupid for trusting you, AKA "the friend you told them not to worry about"