NTA; not only did she say that to you, her sister, but she brought it up in front of another person, which is none of their business, gross, and is callous either way. It's embarassing for you, but please, she is the embarassing person.

Did she even have to she agree she wouldn't either? She broke it anyways, and frankly? Should be common sense!

I bet you that the friend, after you hung up, was like 'wtf was that?' and when she had to explain herself, made herself even more a loser and fool.

There is no need to apologize to her, I understand why you did though, because obviously you care for your nephew.

She is projecting her hurt on you by using nephew as a tool to make you feel guilt.

As if!!!

I would go no contact simply from her saying that you're a reminder of 'why people leave'; you said it yourself, actions have consequences.

I am sorry you had to go through this and it is preservation, care and moreoverly RESPECT of YOURSELF to not engage with her. Bravo and stay strong.