Am I reading some of these responses correctly? Of course it was unreasonable for SIL to ask, or even have a conversation about it!
You do not ask other human beings to put their health, physical and mental, on the line just so you can have a child. That is adherently selfish and abhorrent. Being a surrogate is a decision that the surrogate must make entirely on her own. She must think it herself, mull it over herself, decide herself and then get professional and legal help to make it safe for her. It's not something you just casually ask about like you would have at age 16 looking for a Saturday job down the town.
Plus, what I'm referring to here are surrogates who do this semi "professionally". Women who choose to be surrogates because they want to. Not for money, but it being their call. Something they just like doing whether it's once or a few times for different families. In those circumstances the child typically goes to the family and while the surrogate might keep in touch... she won't see the child again if she doesn't want to. Doesn't hear of "little baby X" in casual conversation. Doesn't have to spend her life watching them grow up, and potentially keep a huge secret.
Being a surrogate obviously has physical risks like any pregnancy, but the mental strain is something else. Nevermind a traumatic birth, imagine having to mentally adapt to being pregnant for 9 months and that baby not being yours at the end of it. Then having to potentially live a lie. Pretend you never carried them. Gave birth to them. The only way you could cope with that is if it was all your own, uninfluenced decision.
Plus, I find the fact that she talked to OPs husband about it bloody disgraceful. Oh, we'll work on the brother to start laying seeds in his wife's head. Fuck off. He isn't the one having to be poked and prodded for over 9 months for someone else. And for him to be UPSET at OP? Give me strength.
OP, do not get guilted into this. Surrogates in the UK are hard to find, I'm sure, but that doesn't mean it's on your shoulders. It's an awfully sad situation for her, granted, but it's not your issue. In surrogacy everyone always talks about the prospective parents. Never the surrogate. It's like they cease to be a human, simply an incubator. But you and your feelings matter. And, to put it bluntly, this is nothing like an organ transplant. That is often a matter of life and death. And you'd still be entirely reasonable to say no. But... having a baby isn't even close to that. Say no, protect yourself and don't ever feel guilty or bad about it.