shower is next weekend and a bunch of my guests have not recv invites

My sister says she mailed them out last Monday.  I have people msg me on FB asking if they were invited.  I called my sister and she told me yes I sent them one.  My step mom received her's friday and my step sister who lives next door has not received hers nor has her daughter who lives less than a mile from my sister.  I posted a huge entry on FB letting everyone know the invitations had been mailed and if for some reason they had not received theirs to please msg my sister or me.  I ask my sister to send out an E-vite via fb just so people who have not received theirs by mail would know date and time and where I'm registered and she just told me that they should be getting them any day now. 

I just got a txt from my step sister asking if her daughter was invited?  I told her yes ofcourse she is invited....but its been over a week and neither of them have received their invites yet.  I have some cousin's on my dad's side of the family who have not received theirs yet either and my grandma was asking if they were invited when I was there yesterday morning.  I stopped by my sisters house and it finally came out that they had mailed most of them last Monday and the rest were mailed out a few days later.  For all I know they are probably in the bottom of her purse right now. 

I have no way of knowing who has and has not received an invitation.  What can I do??