JO SAYS:
It’s really kind of you to consider your sister-inlaw’s feelings, because fertility is such a painful issue for so many people. Here’s what I’d do: I’d speak to her and her partner privately before Christmas and tell them your news. Say you know it could be upsetting for her which is why you wanted to let her know away from the glare of the family Christmas meal. Give her time to come to terms with your happy news.
I think you’d be surprised; most women can delight in the joy of another woman’s pregnancy and you may give her hope that her turn will come.