How to set Boundaries with your family when planning your wedding day.
In terms of fortifying the bride to set boundaries around the things that are most important to her (or them),
I would offer these reminders:
Setting boundaries is not unkind. Consider your boundaries as relational gifts, like a user manual for the people in your life on how to love and respect you. Boundaries take the pressure off of others to be mind-readers.
Setting boundaries earlier (usually) prevents a much harder conversation later when the other person has convinced themselves that there is no line because you haven't set one.
It's easy to say, 'It's your day, so have it your way!" but the reality is that it's often a more complex situation for those people who have loved ones contributing to their wedding financially or with some other form of resources. If this is the case, consider the things that are very important to the contributor and find where you have wiggle room for the things that you don't have strong opinions on. For the remainder, communicate why it's important to you and follow the above steps.
You're allowed to say no.
Remember that everyone carries unconscious material within them: unhealed wounds, fears, resentments, limiting beliefs, unmet needs. And in times of vulnerability, this material can surface and influence how that person behaves. It's usually driven by a need for self-protection. While we can't control other people, we can be aware of our own stuff and take steps to meet our own needs.