You could say it's their traditional role to try and make a wedding "right" according to their own wants rather than the couple's.

Give us some credit. Family's too important to both of us for them to be left out! But please just remember it's our day - we'll always get chances to celebrate being family, but we'll only ever have one wedding. I swear you'll be included, but let us be at least a little selfish with it, okay?

It's already bad enough thinking Grell's gonna try and take control without you two fighting for it.