This idea is not only wrong but also can lead to a great deal of family chaos. Imagine the emotional turmoil it would cause for all parties involved. The hubby would be in an inappropriate situation, and the sister would be involved in something that goes against proper behavior. It can also damage the family structure and the trust within the family unit.
In a healthy society, we uphold values such as loyalty in marriage and proper boundaries in family relationships. Sharing a hubby in this way is not acceptable. It's important to respect the institution of marriage and the relationships within a family. There are so many other positive ways to build strong family connections without crossing such inappropriate boundaries.
Sharing a hubby with a little sister is completely unethical in most cultural and moral contexts. Marriage is a bond between two individuals, and introducing such a situation violates the basic principles of monogamy and respect for family relationships.