Since it's her first, I'd probably say "Well DD really thought some of the more experienced moms would know what they especially liked for there baby". If they still were complaining they couldn't think I'd maybe say "But I know she likes F,G,H,I,J,K,L and the Britax Marathon Car seat" (some people really do need ideas. Like my gma, it's been so long since she's been around infants, or a 2nd cousin, she's 50, childless and really.. well, she brought one of those Leapfrogs w/ extra books for like 8 year olds.. to a baby shower. But the mom and dad liked playing with it
(and fwiw, if she is looking at carseats... I highly recommend the Britax Marathon. My mom got it for my nephew, it goes from like 8 lbs to 80 lbs.. and it's incredibly easy to use. But it's heavy, I'll admit that. It's one of the best crash tested ones. But I figure if Kid- notsofriendly me could use it, it's a great one. Some of them you need a degree in engineering to figure out I swear).