R.M.
A 50th wedding anniversary party is going to last a a lot longer than a baby shower...so I agree with Ellis...go and set up the Bridal Shower...get things started ( talk to the bride ahead of time and explain to her your predicament) stay the first hour or so of the shower then excuse yourself and race to the 50th Anniversary Party. You can do them both..keep everyone happy and not be too terribly stressed out.
The lesson in this is...communication communication communication...lol.