Not unreasonable at all. This little miracle will become part of their family too and Is a cousin for any of their future children. And it's not going to go away..

I would give SIL a call and commiserate with her about how her IVF round didn't work. You know first hand the emotional physical roller coaster that this plays on a woman and couple. Lend an ear and see if she'll talk. But eventually bring up your pregnancy acknowledging how difficult it was for you to tell them knowing what they were going through. That you totally understand if she doesn't want to talk about it and that you'll give her time to get used to the idea...

I dunno. I'm not an IVFer myself so maybe I am not suggesting the right approach . I just want you to be able to celebrate your miracle with your family