Why would that mean you need to cancel a booking?
I think your choices are
1. honour your booking - it's not your guests fault that family members are coming into town unexpectedly. You can either get your family members to bunk up if they want to stay with you or help them find another BnB.
2. Find another BnB for your guests and pay the difference (if they are willing to move).
3. YOu choose to cancel the booking - then you need to take the penalty. - You can't have it both ways 🙂