I’ve posted about this before as a warning to people about inviting poorly behaved children to weddings and in your case OP I think this is something that could happen to you so I’ll share again…
A friend who used to be my hairdresser wanted a child free wedding but her whole family fell out with her to pressure her into inviting her 6 year old niece and 8 year old nephew.
In the end so many of her family refused to boycott that she felt she had to invite them but she was worried they would ruin the day as they were spoilt and badly behaved. Her sister and parents promised they would keep an eye on them but of course they didn’t and the day did end up ruined.
My friend had her best friend as a bridesmaid and made it clear she didn’t want any more or any flower girls but her niece was dressed in a bridesmaid/flower girl dress and had a huge screaming tantrum she couldn’t walk down the aisle. Her grandparents tried so hard to settle her but she sobbed all the way through the ceremony and wasn’t taken out.
Both kids kept pushing each other and fighting and all you can hear when you watch videos of the ceremony is the two of them as they were so loud.
Someone gave the niece a carton of Ribena and she was being annoying trying to get under the brides dress and standing on the train before the photos. When the bride tried to stop her she ended up with Ribena squirted over her hair and dress and that didn’t come out despite trying to sponge it clean for half an hour.
The kids didn’t like the meal and were crying and having tantrums about wanting chicken nuggets. The nephew pushed his plate violently away when being asked to try a bit and food went flying everywhere.
During the speeches both kids started running round so the brides new DH said they needed taking outside, their mother refused so one of the guests ended up insisting on taking them out and missed being thanked in the speeches and being given a gift as she’d made the cake.
The bride asked her sister to take the kids home for the evening ceremony but her mum, dad, grandma and brother said they would also leave and promised they would watch them better so she reluctantly agreed they could stay.
All of her family were more interested in drinking and socialising then watching the kids who were running round getting in everyone’s way and her nephew pulled the edge of a tablecloth to steady himself and pulled all the drinks off the table so there were spilled drinks and broken glass everywhere.
Her bridesmaid best friend ended up spending so much of the evening trying to stop the kids running riot and trying to entertain them by dancing with them or taking them outside, my friend was really upset as she actually wanted to spend time with her best friend and felt guilty her night was also ruined but with no one else watching them they both felt it was the only way to reduce chaos.
The kids had to be stopped from poking the food at the evening buffet and their mum told them to go up first as they hadn’t eaten the wedding meal. They absolutely piled plates high, picked up food and put it back then hardly ate anything.
Her nephew was throwing prawns around screaming how “gross” they were until an older relative slapped his hand away from them and told him to cut it out 🤣.
This caused a big argument with the bride’s parents saying it was child abuse.
The icing on the cake (literally) was when the cake was brought out to be cut and there were multiple deep ridges where the kids had put their fingers into the cake and run them round to scrape off the icing. Lots of sugar flowers and decorations had been removed and eaten.
The bride’s parents had promised they would be responsible for keeping the cake safe and laughed when they saw the damage and said it was “obviously just too tempting for little hands not to touch”.
The friend who had spent days making the cake as a present was in tears as my friend never even got to see it before it was destroyed.
My friend had been gritting her teeth and holding it together till then but ended up breaking down and sobbing and said she knew her day would end up ruined and it hurt her so much to think she didn’t matter enough to her family to just have one day for herself.
There is a bit of a backstory that her parents helped raise her niece and nephew as her sister was a single mother who resented her situation and she felt she always came last after they were born so she felt this proved it.
My friends DH arranged for them to go abroad on their first wedding anniversary to “renew their vows” and they had another small low key wedding ceremony with his parents and sister and a couple of friends. They class that as their real wedding but my friend is very low contact now with her side of the family and they refused to pay for any damage including to the dress, cake, the broken glasses from the pulled tablecloth and the kids had apparently ruined some flowerbeds in the garden of the hotel that held the reception.
I would be firm about not inviting your sister OP or it could lead to a lot more arguments and resentment if you back down and your day is ruined.
If your dad refuses to come without your sister then I’d just say if that’s what he wants then it’s up to him, if you don’t back down he’ll likely come anyway and is just giving you an ultimatum to force you to give in.
I wouldn’t even be rushing to have your sister at the ceremony (the most important part!) as pp have pointed out it’s possible your sister will be wearing a bridesmaid/flower girl dress and shoehorned into the wedding party. Your dad and stepmother will try to make your sister the centre of the day and if they don’t get their way then all 3 will have tantrums and sulk.
There have been a couple of threads about the damage badly behaved unsupervised kids can do at weddings were I’ve posted this before and it’s always those parents who are most insistent they attend and are blind to their behaviour.
Have the day you planned and want, it’s one day of your life that should be focused on you and your DH and anyone else that tries to make it about them is better off not coming, if they are comfortable making demands now then they will be worse on the day!