Choukyou Daikounin

Main character Dick lives in poverty, but happily with his father. However, father unexpectedly dies in an accident leaving protagonist in charge of sister Riz. One day letter about debt repayment comes. Dick plans to deal with it tomorrow, but Riz slips away alone the same day. Dick is not allowed inside the mansion, and Riz is reported to have sold herself to repay the debts. Boy soon discovers that Riz is confined inside against her will and treated poorly. Mansion owner makes a business proposal - he will free Riz if Dick trains girl Remi to such a level that she can satisfy any customer's needs. And the 10 day training begins.

[From vndbreview]