SIL is scheduled to be out of town the week of the baby shower, so she won't be going. Yesterday, we decided to do the "united front" thing and only go if we both could go. I told her, "I'll only go if you do." And she wrote back and said, "Yeah, well, I'll only go if you're going! And only if I'm in town anyway." So then today she told me she's out of town that day, so I'm also off the hook.
So I'll send a gift, say, "Happy baby shower, sorry I couldn't make it...." and that should be enough.