By ushering, you mean you'll be responsible for asking, "Are you with the bride or groom?" and generally be required to be at the church at least one half hour before the service begins in order to greet guests as they arrive.

Also, many couples choose to have their pictures taken before the wedding which if this is the case for your cousin, your time will be restricted even further.

An usher is only slightly more important than the groomsmen from a standpoint of what needs to be done at a wedding, and the usher is a more desirable position because after everyone is seated, you can leave the church for a while without anyone even noticing. For that reason, the usher is a desirable position to be in for Catholic weddings. Being an usher is a nice honor, but I'm sure your cousin has another cousin or friend that can fill in your spot very amicably. Trust me on this one. My first born was due the same day as my brother's wedding (in Iowa) in which I was to be the best man. I told him I could not do it and that I would most likely miss his wedding altogether. Good thing too, because in my rank of happiest days of my life, I am confident that my daughter's birth would place way ahead of Mike's wedding had I been there.

In fact, my list of happiest days goes something like this:
1. my wedding day
2. Anna's birth
2. Sophie's birth
2. Maddie's birth
3. walking to the medal stand at the Kansas Coliseum beneath the crowd noise and flashing lights
4. KU beating Nebraska in football
.
.
25. when my kidney stone finally exited my body
.
.
789. my cousin Scott's Catholic wedding (I was an usher)

In the grand scheme of things, I would tell my cousin that I am flattered for the invitation, but I am not wanting to affect my focus on a goal that I have been building towards for an entire season. I would however, be happy to sit at the guest book for you during the reception.

Good luck!