One thing, can we please not call the Ponds his in-laws? It's not as if that was a real wedding?
While we're at it can not call them the Ponds since they are the Williams'?
They are? Funny, when the Doctor first called him Mr Pond, Rory protested only mildly, then totally acquiesced to the title.
And these days, it's not at all that uncommon for the man to take his wife's name. I personally know two couples who've done this, and know of several others. And hypenated last names are making a big come-back, with not just the woman hyphenating her name, but the husband as well.
My wife and I both hyphenated our last names. And while it's traditional for the husband's name to take the final and dominant position in a hyphenated name, I know couples who are exceptions to that rule. There's a fairly well known fantasy artist named Randy Asphlund, who used to be known as Randy Asphlund-Faithe, when he opted to append his then wife's surname to his own. (They both thought it rolled off the tongue a bit easier than the inverse).
The point is that there's next to nothing in the series to establish that Amy and Rory have ultimately opted to use Williams as their official surname, while it's been repeatedly established that they both have embraced the surname "Pond", to the extent that they both refer to their child as Melody Pond, rather than Melody Williams.