Of course you're upset, Mittens, total sympathy. Infertility is terribly painful. After 10 years of it and a lot of involvement in support groups for it, even compared with having a disabled child, having lost my mother young, having been a refugee as a child and made repeated trans-language moves since, STILL I would say infertility is probably the most stressful experience you will ever suffer. One of the things that will make it worse is the amazing range of stupid, stupid, stupid things the people around you will say and do! And yep, your DH will be one of them. Most men just seem to be crap at handling stress.
You sound like a very sensible person, for example recognizing that your husband might have felt the need to tell his family, so you probably don’t want advice, just moral support. But let me remind you that your MIL telling something to your SIL, presumably her daughter, isn’t gossip on quite the same scale as publishing it in the News of the World (RIP), and that SIL may just not be very good at offering sympathy. Probably no one else knows, and even if they were told, such is normal egocentricism that they promptly forgot again! And anyway, why should you care about what they think? Is it that you feel ashamed? We all do for some reason, but there isn’t any reason to feel ashamed. Try not to yield to the shame.
As far as practical things to do now, I thought Halfling way up there had the best advice. Clearly, your husband need to work through his feelings about his diagnosis. And you are fully entitled to spell out to your in-laws, through your husband or by email or however, that you do not want to be asked anything. Just tell them in plain words that when you are with them you don’t want to be reminded of upsetting things. Since they clearly have no idea what’s the right thing to say or do, this will perhaps come as a relief to them.
And remember this: some day, when it’s all long over, you will look back and marvel at how bravely you navigated infertility. I told no one about it at the time because of that shame. I boast about it to everyone now, because I think I proved I was much tougher than most of the people around me! You are about to find out that you are braver than you knew.