I think that in some women, stress can mess with fertility/ovulation or something. I know I didn't become pregnant until we settled down. (We moved and had job changes a couple of times while we were TTC #1, and it took 15 months.) So that maybe can be a factor. But to simply say that when you stop worrying about it, it will happen, is not true! So I wouldn't listen to your sister. And hopefully, next cycle will be a good one for you. Give yourself time to mourn your lost pregnancy, too. All the best!