Hi. I nursed my now 4 1/2 yr old daughter until she was three and I had a son when she was 2 1/2. They say your milk changes when you are about 4 months preg. My daughter never said anything. Your oldest will be getting a lot of her food from table food by that time, so I would not worry to much about that. I hurt a little if she would play with the nipple, but not when she nursed. My nipples were not nearly as sore with my second as they were with my first!
After my son was born, I continued to nurse my daughter. However, I always made sure that I nursed my son first or I did it together. If you google Tandem breastfeeding you will get some great positions. I did not get engorged because my daughter was very willing to empty a boob on request. And my milk supply was more than enough for both. I did notice that my letdown was huge and sometimes by son would choke from the amount of milk flowing!!
I had a hard time gaining weight though. I only gained 25 pounds and I had to work hard at that. I did not notice a lot of contractions. But my midwife did start checking me at 29 weeks to make sure that the nursing was no thining me out. The day I did go into labor, I could not nurse her because she made the contractions unbearable.
I slowly weaned my daughter by her third birthday because I was tired of nursing two children. I started by letting her sleep on my naked boob and talking about how much I love to cuddle with her (she was only nursing to got to nap and sleep). I felt kinda bad, but by three she really did not need it anymore.
I am still nursing my 2 yr old son and I still love the closeness that it provides. Again he only nurses now to go to nap and bed. But that is a few minutes for me and him and I love it. I will prob wean him about the same age. I am sure I will have a little guilt, like I had with my daughter, but I will survive.
My daughter and I are still really close and I think that is one great benefit of breastfeeding. Good luck.