Pacific - I totally agree with you on the over sterilising of childhood but I still think getting rid (or at least making your best effort to) seriously harmful bacteria that can cause menegititis, NEC, sepsis, and even in a thankfully few cases, death is a good thing.
Getting rid of any, and all, microbes from a child's environment is, most categorically not a good thing, getting rid of Chronobacter Sakazakii out of a newborn's sole food source is definitely a good thing.
I think, if I may be so bold, that your SIL needs to unclench, just a smidge.....