I have family who had a similar situation and handled it as you are suggesting- in their case it has not led to a happy outcome for anyone. It’s quite tragic, really. I apologise for the length of this post in advance. But I’ve seen the damage caused by presuming a parent or step-parent understands the feelings of their child, and has a right to demand that others put their feelings aside.
In that case, the father had been pretty useless as a father and a terrible partner. He suffered a significant brain injury due to his lifestyle (mother had left by then), but prior to that had been very in and out of their child’s life so not reliable, not a good parent and not providing financial support. His family tried to a degree but could certainly have done better- his mother in particular tried to defend him and probably enabled him more than anything. So, not a happy situation. And whilst their daughter did not “lose” her father in the sense that he died, to all intents and purposes, she did lose her father. Very sad situation all round, I know the mother suffered too.
Mother met, and subsequently married, another man- daughter a bit older but not at school- and they have 2 more children together. Eldest’s father became ill when she was about 5, but presence in her life very sporadic before this.
I know the step-father well. He really is a nice man, he has been a good step-father (something his step-daughter has said and never disputed otherwise). The mother is also a nice woman, and I do believe loves and cares for all her children. I think the children are also nice people. As far as I know all children have been well-treated by step-fathers family. But he has always been known as her step-father, she has always referred to him as such. She has always been a step-daughter to him. I think they were fond of each other- certainly appeared that way- and seemed to have a cordial relationship after she left home. So nobody is awful or a monster in this situation. Relationships were not especially frought or difficult. But it still ended in a total mess because certain family members felt that their feelings/view of relationships and sense of duty/loyalty outweighed the feelings and views of the young woman getting married.
When eldest daughter got married she chose her paternal grandfather to walk her down the aisle. They had a relationship which did not seem overly close to her mother, but was very important to her. And I think her mum underestimated how important it was- because she just didn’t understand the bond that her daughter felt and judged how close they were based on childhood contact/her own feelings about her ex-partner and his family (all of which is understandable). I know daughter felt that her mum put more distance between her fathers side than she needed to. As an adult, she now understands that this was because mum was worried about the impact on her (the child) and also because she did not get on well/they could have been more supportive/being with them brought up negative memories of a very unhappy time in her life. But, the daughter had developed more of a relationship with her grandfather as an older teenager/adult, they were now quite close. Mum was aware that they had grown closer but perhaps not appreciated exactly how their relationship had grown- quite possibly the daughter didn’t want to cause upset by giving a lot of details? Who knows.
Outcome was that mum was very upset that step-father not giving her away, hurt that her daughter didn’t feel the way the thought she should about the family she had created and that her relationship with her step-father wasn’t quite as she thought. She felt a sense of loyalty to her husband who had “taken on” her daughter and treated her well (though I think that is something which behoves any step-parent to do- the parent chose you, not the child, and so being a positive presence in that child’s life is really the minimum you should expect to be). I know her mum was angry that the grandfather would be at top table/she’d have to be around him. The step-father was hurt, angry as he felt left out and that he was “owed a bit more respect” as her step-father. Her younger siblings were outraged on their fathers behalf- said it was a slap on the face, “he’s been like her father since she was small” etc. Genuinely seemed not to appreciate that he was NOT her father, and had never been seen or addressed as such.
I don’t think any one of them really tried to understand the brides reasoning. Just assumed she should feel as they felt about her relationship with her step-father and with her father’s family, that she really should see things as they saw them. The fact that she didn’t was because she was wrong, that she didn’t appreciate “how good” her step-father was and wasn’t giving him the loyalty that was owed. Her mum felt she didn’t appreciate how hard things had been for her and it was a slight to her too.
There was not a massive fall out, though it was made clear that her decision was viewed as unacceptable and told “it would damage relationships”, lots of suggestions of how the arrangements should be changed (very graciously the fathers family “could have a role”, but just not give her away). It was largely very polite and everyone went and “played nice”. It was awful. If you’d put it on paper, nobody did or said anything “wrong” but the smiles were insincere/painted on, the atmosphere stilted.
Within weeks there was a bit of a fall out (again nothing dramatic) and relations have soured. Daughter has distanced herself from her mum and family, they are still upset. Several years down the line and they have a child, but very little contact. There is no small degree of bitterness.
I think you should carefully consider why you feel your sister should feel the way you do about her relationship with her step-father and her uncle, and why she should ignore her feelings to keep you all happy.