This. I would more than likely already have plans to do something that afternoon with my mother so I wouldn't attend. Knowing my mother, she'd accept an invitation from her own children for Mother's Day before accepting an invitation to a bridal shower - so she wouldn't attend either.
I'll be blunt here though I may get flamed for it... Honestly, I'd kinda raise an eyebrow at getting an invitation for a shower on a day that's usually spent with immediate family. Let's face it - we can be honest enough to admit that, in reality, showers are about giving/getting gifts for the couple - but there's always that "fine gleam" of having it be about visiting with the new bride and giving your well wishes in person. To have it on a day when a lot of people won't be able to attend (but will, of course, still be expected to give a present) would kind of irritate me and just look like a grab for gifts. Just my $.02.
Maybe you can ask your sister if she's sure there isn't any other weekend they're coming into town for planning purposes - fittings, meeting with the caterers, etc. Otherwise, I'd do as another poster suggested and try to fit the shower in sometime on the Saturday. It would cut their time at their friend's wedding short, but honestly, if they're expecting gifts from people at their own shower, they really should be the ones a little inconvenienced, not the guests to the shower.