"Does anyone else remember that one time in 2020 when your sister, who was your best friend for your entire life, decided to get engaged to your ex-husband?" she said in the clip posted to TikTok.
In a series of videos, she referred to her relative as "trash" and described the situation as a "dumpster fire".
She also said that when she was married to the man, who is 20 years older than her, he did not get along with his then-sister-in-law.
The TikTok user said she found it "weird" that, following her divorce, she did not hear from her sister for a long time.
It was only through social media that she found out the two had become involved.
The woman said she felt "betrayed" and "embarrassed" about her sister's new relationship with her ex-husband.
The original post on TikTok has gone viral, with more than 2.1 million views.
"This is soooo wrong. Can't wait for their divorce," one person commented.
"That's messed up," another one said.
