Well, most arrogant know-it-alls have some insecurities underneath, and that's what causes them to boast about their achievements--because they try to convince themselves that they're worth something.
That may be taking it to an extreme, but give Molly some insecurities. She is the eldest daughter of Percy Weasley, who as we know is quite ambitious. Perhaps she has a hard time living up to her father's expectations for her? Or maybe Molly Sr. still holds some grudges for Percy abandoning the family like that, and Molly isn't very close with the rest of her cousins and is jealous of the close relationship they all seem to have.
Whatever the reason, don't make her shallow and arrogant through and through--give her some depth and likable points, because no one enjoys reading about someone they don't care about.
xx Ariana xx