I also know a mom who always asks that her younger daughter be included in birthday parties that the older is invited to. She asked at my dd's party. My dd had never even met the younger girl but when the mom called to "ask" (more like tell), mom told me the little one had already made the card and she didn't have the heart to tell her she wasn't invited. This was about an hour before the party.
you know i dont really mind this. in fact in all of dd's bdays i have always invited the younger and even older sibling too. some of dd's friends have younger sisters who really REALLY want to go to dd's bday party (they are usually close in age). some know her (with many we are not really friends with the parents but the kids have had enough sleepovers where dd's gains ardent admirers) and some have just met her in school when they came to pick up brothers.
its for that reason i have always included siblings in dd's party. and i have always factored that in the count. i have left the option to the parent. mostly they do end up bringing the sibling/s too
the mom usually stays if she brings a young one.