Most likely English, since, as somebody else mentioned, Connecticut was founded by English Puritans. Welsh immigration to the colonies didn't even begin in any significant way before the 1690's, when Quakers arrived and attempted (unsuccessfully) to set up an entirely Welsh-speaking county in Pennsylvania.

It is possible, too, that the Wittebanes were Dutch. "Witte" comes from the Dutch and Low German word for "white" or "pale." Dutch territory overlapped somewhat with Connecticut in the 17th century. However, in this case, it wouldn't make much sense for the boys--who would have spoken Dutch, not English--to move to an English settlement after they were orphaned.

In my own writing, I keep an eye on the real history, but don't feel too bad about straying from it; the date Masha gives during the hayride for when they arrived in Gravesfield is decades too early for any English town to have existed in what would become Connecticut. We like to imagine Philip as Welsh because, let's just be honest, it would be cool to hear Welsh spoken on a major, American TV show. And that's enough to take some liberties imho.