If Vetch is in this book to show us how a magician lives when he's not being tortured by his own pride and his own shadow monster, then Yarrow and Murre are here to really drive that point home. Yarrow and Murre are not only Vetch's sister and brother, respectively, they're also examples of normal people living normal lives full of normalcy. Normal normal normal. (Or at least, "normal" for living in a fantasy world.)
But, even though they're both normal, they're in this book for different reasons. Murre is a boy without magical powers – he's perhaps what Ged would be like without magic. Murre is the same age as Ged, but he's totally different from the tortured magician. Here's what the book says about Murre: "his life was easy and untroubled. Ged watched him with wonder and some envy…" (9.55). Also, a "murre" is a type of bird, so there's a direct comparison between Murre and Sparrowhawk: they're both named after types of birds, so it extra makes sense to compare them. But curiously, a) the boys are totally different and b) they totally envy each other (Ged wonders what it would be like to live normally, and Murre wonders what it would be like to live heroically).
For Yarrow, the case is a little different – by which we mean totally different. She's less the alternate form of Ged (like Murre is), and she's more like what he's missing out on. Like, if he were a normal person, he could marry and spend all of his time with her. At least, it seems like that's where the book is pointing us; it's never said explicitly, but do you feel like there's something romantic going on between these two? Maybe it's just that Ged guesses her true name, which is kind of crazy special (10.14). Also, the way she stands on the shore to see them off and is happy to see them come home – that makes it seem especially sentimental to us.
Of course, Ged's too tortured right now to date, but maybe in the future, he'll go out on a double date – if his shadow can find someone to go out with too.