Immigrant visa petition - changing applicant number

Hello, I am currently filing for family preference immigrant visa. My uncle filed the petition and my mother is the principal applicant. My family's plan was to immigrate together, so in total four people is listed as applicants(me, my mother, my brother and my father). The problem is, after the interview at the embassy, we received a blue letter requesting for joint sponsor's financial documents. We think it means that my uncle's income is not enough to support four people. We calculated the income and found out that it was enough to sponsor two people, instead of four. In this situation, can we withdraw my father and my brother's application so that my mother(principal applicant) and I can immigrate to US? It is because my school in US starts very soon and all the tuition has already been paid. And, we do not know anyone who can be our joint sponsor. If this is possible, how should i notify the embassy? just a letter indicating that my father and my brother wish to withdraw?