Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.
For example, "We chose the facility based on the needs, that decision is firm, and we are now focusing on other issues."
Controlling people like to find holes in your resistance, and they see changing words as a chance to exploit things.
It may help to give her some small self contained part of the project to work on. (Or it may give her a chance to try to make that part grow.)
My opinion is that things like a big party for aging parents is that it is to be controlled by blood relatives and "in-laws" need to take a step back and assist as allowed. Have you tried telling her that "If this was a party for your parents, my daughter and I would follow your lead; we request that you do the same!"? Likewise, keep your voice calm, etc etc etc.
Have you appealed to your brother to help reign in his wife?
Gentle hugs
On planning an event like this there are plenty of areas she can assist or be included in. I am not a fan of the use of "We have..." or any qua-say exclusionary wording. Since as you say the next generation has made the party plans. You can deflect by complimenting her choice but say that you fully support the decision of your daughter and first cousin. Though you would like to keep the idea venue for the future sometime.
I am sure they have made a list of wants for the occasion.
Another option of inclusion might be the personnel support of preparation. Something like a outfit shipping for the event, hair appointment or mani -pedi....these might help her concentrate in her presentation in a inclusive camaraderie.
Of she does any crafty things...maybe hand made cards that are at each place setting so that the guest can express their thoughts and love of the couple, Things like questions of secrets to long lasting companionship? How they may have inspired the writer? etc...
Or collect photos/videos off the social sites/ email of guests that would truly like to make it but cannot and create a video or collection of the distant well wishers, for the couple.
Ideas are popping in my head, but I know great planning takes prep and delegation, and sharing the load helps people feel included.
J.