Family business owner retiring

I have been involved in a family owned and operated business for over 20 years. I myself am family of the owner by marriage only. I have been presented with the opportunity to become a partner in in the purchase of business. My question is I am not in a position to go to the bank and carry a loan for my portion of the business, I am being told I would be able to purchase my portion of the business with being there for knowledge for day to day operations. My question is how can I go about making sure that I can protect myself in the event the money partner wants to sale or get out how can I protect my knowledge investment of the business. Please keep in mind I am not real up to par on the legality of business law or admin I have been out in the actual day to day operations of the business.