You will want to create an "amendment" to your partnership agreement.
Unfortunately, the changes in your partnership agreement will create some more complex changes than what are dealt with in the normal scope of a simple amendment (your sister has died, and, unless she had a trust, her interest in the partnership should have been distributed through probate (either a small estate affidavit or full probate depending on the value of all of her assets)).
If neither of these has happened, you really will want to hire a lawyer to assist you with this.
If your sister's interest was properly passed down to her children through probate (or a trust), you can use an amendment to document the other changes (the other sister can sign her interest to the brother, and document that transfer).
I still do recommend hiring a lawyer, and I would suggest looking into another form of business entity (such as an LLC) which will not only make management of your business much simpler, but also provide each of you the benefit of limited liability (so if there is a loss to the business, your personal assets will not be at risk). Again a lawyer can help you discuss these benefits and costs, as well as help you draft a set of governing documents that best fits your needs (you can find free documents online, but these are simply regurgitations of the state's default statutory rules, so while you will get a legally valid entity, you will not get the full benefit of the flexibility and protections - nor will you get the benefit of advance planning for potential disputes or dissolution of the business - that you would if you pay an attorney to draft these documents for you, it costs a little bit of money but it is a solid investment).