Is it right to make a Christian sister the church secretary? Can a sister also act as the treasurer?
First off, you will not find a position of secretary or treasurer mentioned in the New Testament. The closest you will get to a treasurer is that Judas used to keep the money box (John 13:29). The closest you will find to a secretary is that Tertius wrote out Romans for Paul (Romans 16:22). Thus, the tasks have precedence, but there were no official positions in the church.
Typically a deacon takes care of the finances for a congregation. Since keeping the books for a church is not a position of authority (I Corinthians 11:3), nor does it requires addressing the congregation (I Corinthians 14:34-38), it could be done by a sister. The same is true of a secretary. Most congregations are have no need for someone to handle secretarial duties, but for the large congregations there is no reason a woman cannot deal with the tasks that need to be accomplished.
You do find precedence for a woman doing tasks on behalf of a church. "I commend to you our sister Phoebe, who is a servant of the church which is at Cenchrea; that you receive her in the Lord in a manner worthy of the saints, and that you help her in whatever matter she may have need of you; for she herself has also been a helper of many, and of myself as well" (Romans 16:1-2).