Why do they think they should benefit from YOUR INHERITANCE? And why do they think you OWE THEM A PLACE TO LIVE AND TOTAL SUPPORT FOR THEM? NO, JUST NO. Sis and the folks need to learn YOU do NOT OWE THEM for ANYTHING.
How long do you need to LIE about your beliefs? Did your parents get a confirmation? Just tell them you are not buying it and move on with the conversation. You do not need to argue the point. JUST MOVE ON.
Talk about entitlement. After your Mom died your dad had no more reason to be involved in the business. That was a business with your MOM AND GRANDPARENTS. As for SM she needs to back off and realize she is not and never has had a say in the business OR THE MONEY LET FOR YOUR THREE BOYS, PERIOD, END OF DISCUSSION. And you do NOT need to cater to half sis about this either. Her mother is putting her up to getting BOTH of them IN THE DOOR. And if you did THEY WOULD TAKE OVER COMPLETELY. DON'T LET THEM IN.