If the alleged offence is compoundable under Section 320 of the Criminal Procedure Code (crpc) then the court can permit your sister in law to withdraw her case.
There are two kinds of compoundable cases defined in the crpc. One without the consent or leave of the court and another with the leave of the court. In the former condition the presence of the accused is not mandatory at the stage of withdrawal of case.
In the latter situation the court can issue summons to the accused for his personal appearance in the court. The accused should appear in the court in the pursuance of summons.
If the court does not issue summons in the latter situation it deems that the court has dispensed with the personal appearance of the accused. Hence, the appearance of your brother in the court proceeding will be decided on the basis of nature of offence and intention of the court.
Also read: Quashing of non-compoundable case