Tell her she is a big girl.
I did extended breastfeeding... and my kids self-weaned.
My Daughter self-weaned at about 2.5 years old.
I talked with her about it.
I only nursed if she asked.
Or if she asked, I would say "In a minute, I am busy..." then I would NOT sit down.. and made myself busy. Then she would forget about it.
I taught my daughter 'manners' about it... ie: it is my boobs, she can't pull up my shirt, she needs to ask nicely & not demand it, only at home and not in public etc. It was fine... she understood. She was mature enough.
Then you also have a baby to nurse... so you can explain that baby needs to nurse and he is younger... and does not eat food yet like her... SO.. baby can only nurse from Mommy. But SHE is a big girl already and she CAN eat food... so that is why. That is what 3 year olds do etc.
Then I have friends who did extended breastfeeding... then they wanted to wean their child. So they put Band-Aids on their nipples, and said "Mommy's milk is broken..." or "Mommy has a boo-boo..." and they said this work WELL for them. No problem.
But, you have a baby to nurse, so that won't work for you.
Ultimately... she is responding that way because there is a baby in the house. So you have to address that... NOT just the nursing. She needs help.. in ADJUSTING to having a baby in the house. "She" had a baby too... not just you. She needs to adapt and transition to that... and her place in the home and the family. Because... her ENTIRE life changed... once the 2nd baby came home. So... address that.... not the nursing specifically per say... because, there are other things.. that she is needing to adjust to, too.
My daughter self-weaned. One day she just told me 'I don't drink from you anymore..." and then she giggled. And that was it.
all the best,
Susan