She said when she grew up, it was considered rude to have mother/MIL throw the shower because they would be benefiting as well.
The idea was that since families should take care of their own, the mother throwing her daughter a shower was no different than the daughter doing it herself. (If the daughter couldn't afford it, the mother would have to help, so throwing a shower to "avoid" that obligation was rude.) Obviously this wouldn't apply to a cousin or a sister-in-law, since that relationship carries no dependency burden, so I wouldn't agree that showers wouldn't have been hosted by *any* family - just that they wouldn't have been hosted by family on which the MTB or FTB had ever been dependent.
Nowadays, with families more spread out and less reliant on one another, the no-mothers-hosting rule may be a little outdated. Still, my family is so close - physically, financially, emotionally - that I would be very uncomfortable having my mother host my shower. I know she wouldn't even consider doing so. My shower(s) have been / will be thrown by people like god-parents, old family friends, cousins, etc.