When to call guests who haven't rsvp'd?

I'm throwing a surprise baby shower for my brother's wife (sister in law). My problem is that only one person has rsvp'd yes. There were 3 maybes a TON of guests who haven't rsvp'd yet. Very few actually even viewed the invite. I asked around prior to see if anyone was throwing her one and not even her mom was- what gives?? I tempted to send out a followup email this week and maybe invite a few friends who knew my brother growing up. Their wedding was last year and had a great turnout- so I'm confused on the non-rsvps. I also don't want her shower to be a disaster... should I cancel?