It’s completely okay for you to fall in love with anyone. If your boss’s sister wants to have a relationship with you and you also have feelings for her, then you should go for it. You should think about your happiness first. You shouldn’t think about what her brother will think or the boss will think. He is your boss in the company but he doesn’t have any hold on your life. You should make a decision on your own without worrying about others. If you are sure you also want to start a relationship with that lady, then go for it without thinking twice. You shouldn’t worry about your uncertain future while ignoring your present. And your boss would also have the sense to keep his professional and personal life separate.