At that age you can set limits to her nursing. She's old enough to understand.
I am tandem nursing my almost 2 and a half yo DD1 and 5mo DD2 and since DD2 was born, nursing DD1 is not as pleasant. She was nursing all the time and I was starting to resent her nursing but I felt so guilty that I couldn't say no. One day my dad felt the resentment and that's when I realized I needed to do something for our relationship.
I sat her down and told her that mommy is tired and she's a big girl and from now on she'll nurse 3 times a day. Once in the morning, once after nap and once before bed. It was easier than I thought. Everytime she asked to nurse in between I would repeat that and she would wait.
I decided to cut the amount of nursing sessions but you could always cut the length if you prefer. You could tell her you'll nurse while I sing the ABCs song and then stop.
Whatever you decide, be consistent, that's the key