Did the co-founder actually state that the reason he/she deserves more is that he is related to the investor, or did he/she state that the investor would not have invested without them.

If they said the former, you shouldn’t partner with then you know you have a problem. if it’s the latter, you should go back to them and ask them to get the investor to state that this is what they wish. In fact you should raise it with the investor.

It is likely that the investor would not have know that a step child is behaving in such a way, and if they are a savvy investor would take him aside to tell him to differentiate between business and family. if they insist, you know you are being taken for a ride. Take your skills and bring it to another investor and partner.