Whatever arguments you may be guided by the arguments that you cite as your reasons for failure should be thought out, measured and formulated so that the employer felt you review your failure and his position too. To prepare and failure to argue, take a time-out.
To assess the effects of the transition from the point of view of the employer. Great motivation will be you expressed confidence that in the case of your transition to another position, you will have to leave work for tasks that are very important to the company. Explain to the employer that the search for a replacement for you will require a lot of time and be quite costly for the company.
In that situation, when you want to refer to the inability to accept the offer due to the fact that at the moment you need to solve some family problems, don't start talking about them. First thanks for the trust and say that the proposal you are very interested. Show how much you are aware of the situation and that you are aware of the policy of the company. Then say that you have analyzed their possibilities, but can use them only after some time, because at this stage you are faced with an important problem in the family, for which requires your participation. Such refusal will not be taken as an insult and you will simultaneously show yourself as a responsible person will demonstrate the makings of a future leader.
It also happens that you do not like the direction that you are going to charge. In this case it is reasonable to say directly about this, thanking for the trust. Share your plans and consult with the head, will it be possible to bring them to life.