Just a tiny bit of advice that I hope helps this situation. You and your husband must now, above all else, stay united and support each other's decision to pursue Orthodoxy. It's especially hard for your husband who may understandably feel like he's turning his back on his parents, but if you show weakness or waffle in your remarks to them, they'll sense weakness and an opening and go for that like a lion weeding out prey. Sorry my analogy sounds a bit grisly or militaristic but this is really a fight, both for your free will and independence as a married couple and your spiritual lives. I don't know the age of your children, but young children can sense when parents aren't united and they are also looking to you for a smooth and natural transition to their new church home.
If you haven't already, reach out to as many folks in your Orthodox church as you feel you know well enough, and ask for their specific prayers. Ask any converts in the parish if they had a similar experience. Just the sharing of convert stories can make you feel better.
And with regards to your FIL, well, anything is possible. He may come around eventually, but be prepared for the possibility that he may never be spiritually mature enough to accept this. God asks us to love our parents but that doesn't mean we have to like them or their decisions. Give yourself a break feeling guilty over this.