So sorry for you loss Hun

I don't want to hurt or offend you, if I do let me know and I'll delete my reply but my view is from the other side of the fence...
Are you sure they didn't have plans if trying before you lost your precious bub? My SIL just m/c, and were ttc #2 at the moment. When we found out about her loss a few days ago I felt guilty for if we conceive soon but we can't put our family plans on hold forever because a) we don't know how long it'll take her to conceive, b) we don't know how long it'll take us to conceive and c) we want our kids close in age for our own reasons. Your step sister probably expected it to take quite some time to conceive, and possibly didn't tell you herself because she cares and didn't want to hurt you.

In saying all that, my point is she probably wouldn't have done it to hurt you, even tho it might seem that way now. It'd be too much to expect you to see it from her side just yet as you need your time to grieve, but go easy on her, every pregnancy is a blessing, maybe just ask her not to talk about it too much in front of you for a while.