I am on very good terms with my exDH who is thankfully not a sulker (just very very dull and a bit manipulative) but my exMIL is a narcissistic gaslighting bigoted sulker who I have not spoken to for 8 years now.
Like many PPs, I found that she sulked or had a strop when it was a special occasion or something important to me. Several times exDH would let her in on a surprise for me (sometimes because he wanted her to babysit) and she always spoiled it by telling me the surprise. On the morning of our wedding day she phoned me and told me the fancy hotel he'd booked for our wedding night 🙄
When she wasn't being a bitch, she pretended to be my biggest fan, always banging on about what a great wife/mother/person I was. And I was taken in and did so much for her, thinking it just took time to get to know her.
Christmas 2006, she took offence at me asking her to bring the kids' presents with her when she came on the 27th, rather than delivering earlier, so that they weren't overwhelmed and could have fun opening them with her. She emailed me, saying "Christmas is cancelled. If anyone asks, tell them I'm fine, but apparently not dying as quickly as some people would like. I hope you'll be very happy without me"
Of course, she then turned up on the 27th, with presents, like nothing had happened 🤣🤣
During lockdown, she sulked because one of my teens didn't want to join her "family" WhatsApp chat group (basically her and her sister) - she emailed him (aged 16) such a nasty rant about how ungrateful he was. He was a bit bemused but when I showed it to exDH, he "had a word" and she emailed again saying "please excuse that email, I've not been myself recently, I think Covid affects old people differently". He now barely sees his grandmother because she's a weirdo (his words)!!
I can laugh now, but sulking adults who are supposed to care about you is damaging and toxic. Hurrah to us all who have got rid 🎉