Can my siblings sell their ownership to others or if they aren't interested in running the business and do they have to ask me to buy them out?

The ability to sell an ownership interest in a business depends on if there is a restriction on transfers. Is there a buy sell agreement that restricts transfers? Is the business organized as a corporation with transfer restrictions on the stock. Is it a partnership with a partnership agreement that restricts transfers. If there are no transfer restrictions on the ownership interest then you siblings would be free to sell their interest to whomever they choose. You have not provided enough information about the nature of the business and how it is organized to provide you with an answer.

Depends on whether there is a shareholders agreement signed by the parties. Typically in small corporations, the shareholders sign an agreement which limits the rights of a person to sell to third persons. If there is no such agreement, then they can sell the stock to anyone.