Mother In Law and Infertility

So my husband and I just saw an RE and we are facing some pretty difficult male factor infertility due to a condition that my husband was apparently born with. We have not told anyone anything about our infertility even though we’ve been TTC for 2 years.

Now that we know the problem, and there is a treatment plan in place, we are considering telling our families. My husband doesn’t want to reveal to them the medical condition that is the cause of our infertility. I think it’s because it makes him feel like less of a man that he cannot produce children naturally. I have been super supportive of him, reminding him how much I adore him, and explaining that I would stay with him no matter what.

However, I do not have the greatest relationship with his mom. She has pestered me on several occasions about why we haven’t had children. I feel like she thinks that I’m preventing her son from having children. She even vaguely asked me if we were even having sex! Like I’m not giving him any? (Keep in mind we have had a very active sex life for 10 years). How can we tell her about our infertility without her blaming me?! I want so badly to tell her that my husband was just born that way, and that there’s nothing I have done to cause this!