2. As for child custody, it usually goes to the person who wants it. If they both want it, it usually is split. Rarely ever do both parent not want their child. However, if both parents do not want custody of their children, they can be and will be sent to foster homes.
3. Spousal support is to be paid by the person making more money to the person making less money. Gender does not matter.
4. If your sister-in-law must get a divorce, there is nothign to prevent it. As such, it is best to sit down and negotiate the terms of the divorce, such as child custody, child support, spousal support and division of assets and liabilities. Maybe an attorney's help as well.