While selecting a job candidate, HR or employers look at the candidate's suitability of the position, competencies required and then decide the salary. Secondly, position that you apply for also matters. Past salary is one of the factors to decide the remuneration but that is not the only deciding factor.
Since you have worked in your father's company, you must have got exposure to various aspects of business like operations, finance, purchase, marketing etc. Utilise your knowledge and put up absolutely confident posture. Rather than driven by HR, rather than nurturing the imaginary fears, command your 'x' salary confidently.
All the best!
22nd February 2015 From India, Bangalore