Hi A.,
It's been many years since I was in your position, but I was still nursing my oldest daughter when I got pregnant with my oldest son. She nursed all the way through my pregnancy, even as the milk disappeared. Some children will naturally wean themselves because the milk changes flavor, then begins to disappear as your pregnancy progresses. But not my girl! I chose not to wean her, but to let her wean herself - she didn't. So we entered the world of 'tandem nursing'. That's the proper term for 2 sibs nursing at the same time. She was delighted (and grateful to her baby brother) when I came home from the hospital with her brother and discovered that he had made the milk come back! There's a lot to consider in tandem nursing: it can be exhausting, you may be 'touched out' by the time your husband gets home & night (you begin to feel occasionally that if you were to suddenly need a mastectomy, your entire family would fall apart!), and it becomes difficult to nurse discreetly. But the benefits are enormous - you'll see your children bond nicely (my daughter used to stroke her brother when they were both nursing together), sibling rivalry is lessened because your older child doesn't feel so displaced - you'll still have an extra arm to hug and nurture her while caring for your baby. She won't need to fight for your attention because it will be easier to attend to both at the same time. It's not for everyone, for sure, but it worked well for my family. My daughter did finally wean - 9 months after her brother was born. But she tapered off slowly at her own pace, until one day I realized it had been weeks since she nursed.
My son, on the other hand, weaned when I was pregnant with his younger brother - he didn't like the changing taste of the milk, and tapered off as the milk supply dwindled. He was completely weaned when I was 8 months pregnant.
Good luck with this very personal decision - certainly one you should make yourself, not based on doctor opinion. There is good information on tandem nursing available through LaLeche League and in the book "The Motherly Art of Breastfeeding".