In context, it refers to the family which is now called Walderhurst.
Here's a longer quote from the passage you're asking about:
She cherished a touching secret desire to know what might be discoverable concerning the women who had been Marchionesses of Walderhurst before. None of them but herself, she gathered, had come to their husbands from bed-sitting rooms in obscure streets. There had been noble Hyrsts in the reign of Henry I., and the period since then elapsed had afforded time for numerous bridals.
This is saying that the family of Walderhurst, formerly Hyrst (perhaps Hurst in between), has been counted as nobility since the reign of Henry I - namely, the early 1100s - and that due to their long pedigree, they have always been married to women from other wealthy families, not a single one from "bed-sitting rooms in obscure streets".
Hat tip to @Spagirl for pointing to this in comments; but as she said she wasn't going to post an answer, I went ahead and looked up the passage in order to do so myself.