Poor your sister, Michaela! Fibroids are actually pretty common, but they go unnoticed most of the time - it's something worth checking out, if TTC seems to be taking a while. I found about mine because I lost some weight and realised that the lump in my tummy wasn't, in fact, my spleen . How serious they are really depends how big they've grown, and whether they're on the inside or the outside of the uterus - inside, they take up room a baby needs and are trickier to remove because you need to cut into the uterus wall, outside, they can either be snipped off the stem they grow on, 'embolised' by injecting a little block into the vein that feeds them, or 'shelled' out from just under the outer layer.
I had the full myomectomy, which is a fairly routine operation, but there's a 1/100 chance that if the bleeding becomes too much, the surgeon has to perform an emergency hysterectomy, so unless it's causing problems (interfering with implantation/painful periods, etc), most people just leave them, and they often shrink after the menopause. Not really sure how much diet can affect them. I put my operation off for as long as I could, and havered for ages about the risks but everything was absolutely fine (apart from the hospital food; if anyone's thinking about having their baby at St George's Tooting, I strongly recommend a packed lunch).
Sorry about fibroid lecture, everyone else. Before I was a pregnancy symptom expert, I was a part-time fibroid specialist.