It’s my feeling, but I want to run this past all you guys before I reply to my friend, that this is just impossible because:
- The Middle Ages had no concept of adoption in the sense that we now understand the word. Certainly you could take in a child, whether a blood relation of yours or not, and bring it up; but you would introduce this child to people as “This is Matilda, who’s like a daughter to us” or, “our adopted daughter”. If you said “This is our daughter Matilda” you would be lying. You’d only do that with deliberate intent to deceive (e.g. if you couldn’t have children of your own, to keep property away from your next-of-kin).
- While you might bequeath such a child property in your will, s/he would have no automatic right to inherit anything from you, or from anyone else in your family. (In fact, in English law adopted children only acquired such a right less than 20 years ago.) And you had no right to leave them anything that was explicitly family rather than personal property.
- This character’s husband would actually have a very good ground for annulment of the marriage, since she is in a fundamental sense not the woman he thought he was marrying. He has married beneath himself, and any children they may have, being the children of a peasant bastard, will (a) have no claim on the De Blank property, and (b) not be eligible for knighthood, the priesthood, or prestigious marriages.
Edited to add:
Thanks everyone for your input: very helpful! Armed with your comments I went back to my friend, who was v. dismayed and said she had been sure the adoption thing was authentic, as she had cribbed it from a published novel. She showed it to me, and so it proved:
Well, we knew there's nothing too rubbishy to be published somewhere, but to have it hailed as "a worthy heir to Ellis Peters".......Gack!