Hello, I nursed through pregnancy and am now tandem nursing my 4 mo old and 2 year old. Nursing through pregnancy was brutal at times, with sore nipples, and just a feeling of "ugh." but I thought it was worth it to just grit my teeth (and believe me I did grit my teeth!) because nursing was (and is) so important to my older daughter. She was showing no signs at all of being ready to wean.
However, I did start to set limits on her nursing- I implemented nursing "rules" one at a time- things like she wasn't allowed to touch my opposite nipple anymore while nursing, and when i felt all done I would count to ten and then she would have to stop and just cuddle, and "we only nurse at home." At first she was angry about these limits, but pretty soon she did fine with them. When the baby was born, I was so happy that I didn't force her to wean cause I think it really added to their bonding. There's nothing like seeing the older one lovingly reach across me to stroke the baby's cheek while they nurse :)
For the first couple of weeks, I didn't put any limits on the older one nursing so she would get used to the baby and still feel secure. Then slowly i started adding back more limits- shorter nursing times, and things like "last nursing till bedtime." Again, she resisted at first but soon was fine with it.
I definitely suggest the "Adventures in Tandem Nursing" book by Hilary Flowers. It's great and gives so much good information. That being said, there are definitely times when I wish that my older daughter had weaned already and that I could just nurse one. But I had to really think about what was best for her and how much she could handle. Knowing her temperament and how important nursing was to her, it just didn't seem right to wean her before she was ready.
GOOD LUCK!!!!!!!