Praise be to Allaah.
What your husband’s sister said has no basis in sharee’ah. There is no reason why a wife – or a sister, mother or daughter – should not be a business partner of her husband.
The basic principle concerning business dealings is that they are permissible, and the basic principle concerning partnerships is that they are open to all. Whoever says that they are not allowed has to produce evidence to prove it. Whether the wife enters the partnership by means of paying a certain amount of money or she is given some of the shares in the company for free, it is OK for her to be a partner.
And Allaah knows best.