WOW!!! That is some request! Please, please, please do not let this pressure influence you. It is very sad that they are having trouble conceiving but as others said, there are so many children that need families, have they looked in to that?
I have never been in this situation or known anyone so I hope you don't mind some advice from the peanut gallery. My heart hurts just thinking about this. What does your gut say when you think of having another child and giving it to them. What do you think of right away? That is your answer. Don't let the sadness of their situation influence you or pressure you. It is not your responsibility to to give them a child.
One wrote about what your children would think of seeing their brother or sister given away, that really makes so much sense. I have to think that their would be life long missing piece to your family if you had a child and gave it up.
Only you can decide if this is right for you, but please put all the pressure aside and dig deep inside yourself and ask yourself if YOU could live with letting your child go. There are other options for your brother-in-law! Take your time this is BIG!
Take care & good luck!