Girlfriend of five years. Clearly a steady relationship. And you think that counts for nothing ? Just because they don’t live together doesn’t make them any less a long term partnership than anyone else. Have a read of OP’s update as to what she was doing at the time of the wedding. Turning the clocks around so she wouldn’t know what was happening and trying to keep busy. That suggests a lot more than a simple girlfriend/boyfriend relationship. It suggests one partner in a committed relationship recognising that the only winners here would be the relatives, and doing the right thing for everyone except themselves, despite being deeply hurt by the decision DD and DP could see little choice in making.
It really wasn’t a question of anyone ‘trumping’ anyone else. The relatives waited until the last minute to issue an ultimatum that they knew was going to be hugely damaging whatever the bride decided. It was either going to damage their relationship with their niece, should she go against them, or damage her relationship with her father if she caved to their demands. The fact that it would in all probability drive a wedge between him and OP was likely an added bonus as far as they were concerned, as was the satisfaction of seeing the fruits of their labour - the widower of their sister attending the wedding alone.
So nobody wins against the spiteful relatives because their demands were designed to cause maximum damage in any event. And the only way to keep them happy was at OP’s expense, sending her a clear message that she is not important enough to be a consideration.
In the aftermath l think both DD and her dad are going to come to regret the decision. For DD that will be when the stress and excitement of the wedding dies down and she realises how these two pieces of work manipulated her special day for their own purely malicious ends. And if OP decides not to continue the relationship with her dad, DD will carry some sense of guilt for that too.
DP will come to regret it if OP decides she can no longer carry on the relationship because despite a five year relationship she was left feeling that no one has her back, and that she’s some dirty little secret to be hidden away from ‘real’ family, and not worthy of consideration. And if she continues the relationship, every time the wedding comes up in conversation it will be a sharp reminder to him that he and OP have no frame of reference in which to discuss it because there are no shared memories of what was a significant event in his life.
I don’t think DD and her dad had any option but to comply with the demands if DD wanted these people at her wedding. And l think OP recognises that. But it doesn’t change the fact that her feelings were the last consideration, or that potentially it will keep happening at other family events.
I would hope that at some point in the near future her DP will let his brother and sister in law know exactly what he thinks of them for so cynically using his daughter’s wedding to express their displeasure at him daring to move on with his life, and for causing so much damage as a result. I would also hope that he has the balls to stand up to them if they try to cut OP out of future family events. Because if not, the relationship is doomed. OP can’t live her life being hidden away simply because they don’t want to acknowledge her existence.