Don’t you want to know where the words for what we call our family come from? No? Well, I do, so tough.
Mother first showed up as we know it in the sixteenth century. Before that, it had a d instead of a th, although the pronunciation was still th (yes, really). It comes from the Old English modor (anyone else see that as Mordor the first time?), and before that, the Proto Germanic mother and even further back, the Proto Indo European mater, all of which had the same definition we had. Not much of an evolution here. Mom, mommy, and mama all share a common origin, going all the way back to the Proto Indo European ma.
Father comes from the Old English faeder, and before that the Proto Germanic fader. It also can be traced back to Proto Indo European, but in this case the word was pəter. The p to f change is common in Germanic languages—common enough that it has a name: Grimm’s Law (among other common letter shifts). There are other words for the male parent, too, like papa, which has French and Latin roots, and dad, which is so old (possibly even prehistoric!) that no one knows where it comes from.
Sister showed up in the mid thirteenth century, coming from the Old English sweostor/swuster and Proto Germanic swestr. It too can be traced to Proto Indo European, where it’s the word swesor. The Online Etymology Dictionary actually calls this one of the least changing words from all the way back to PIE. I just find that hilarious that that’s pointed out. It’s also thought to be a combination of two other Proto Indo European words, swe-, one’s own, and ser, woman. It actually makes sense, right?
Brother comes from the Old English broþor (þ is thorn, one of the forgotten letters; it’s pronounced like the th in math, meaning it’s pronounce exactly like brother). It can be traced to the Proto Indo European bhrater, and just like sister, it has hardly changed over the millennia, which is pretty impressive. PS. Fraternity actually comes from bhrater, too.
Okay, there are a lot more words to do and this post is getting kind of long. I’m guessing there will be another one next week, with more family etymological fun : ).
Tony Jebson’s page on the Origins of Old English