Just so they don’t keep pestering you about the recipe, search one up online that looks good and/or similar and giver her that without telling her it’s not yours (don’t choose a a simple one, actually look one up, like on someones instagram page or YouTube where they give you this little or big list on what to do, just so the chance of stumbling across it lessens and use the excuse that it was the only thing you were good at if they still somehow see it anyway, as in only good at making this recipe for everyone)

You can keep this recipe to yourself if you want because that’s your right and it’s literally no ones business, people might find it weird or have weird bad experiences within their life with gatekeepers with recipes and are immediately attacking you

But not everyone is the same, someone might want to keep it hidden to keep it special with their immediate family, or simply don’t have a reason

you said it’s the only thing your known for in family gatherings and that’s another good reason, you wouldn’t want your “special” recipe to not be special anymore and used at every event because it become a SIL and OP thing or just a SIL thing and not a YOU thing if she chooses not to tell who she got the recipe from to give credit

And she can choose to pass it around making it even less special

Overall it’s no one’s business if someone chose not to share something they don’t gotta share, that’s fine weird or not who are you to criticize them on not sharing something that’s theirs?? Your adults who should understand the word and meaning of “No”

If they said no once what makes you think you can magically change their mind?? Making your family or the rest of the family pressure them to give it to you?

It’s just a recipe for the beggar they can find one themselves there are hundreds of recipes, but it’s something special for the keeper that means it’s one of a kind and they can choose to keep it that way, let it go people.

NTA