Dear Paul,
There is a lot of other information that would need to be taken into consideration in order to answer your question well (like, if your mother is still living, the family economic situation, the exact condition and needs of your father), so let me just give you three quick points you can use to examine more closely your own situation in order to make a good decision.
First you have all the practical questions to consider how much your family actually needs you for their material support, other real sources of income, debts and mortgages to be paid, superfluous things that can be eliminated, what your siblings are capable of doing, etc... Obviously, if they really and truly need you, you have to stay and help them.
Then there is the spiritual side of your dilemma. How many of the needs you see (especially as regards emotional support) are real and of greater importance than the good you would be called to do in a vocation? To put the reality of these needs into focus it may help to imagine what would happen if you were planning to get married and this situation arose, would you need to postpone your wedding? If the answer is no, then there is a good chance that the same answer applies if you are thinking of a vocation.
Another question to ask yourself, it is hard but it needs to be asked, is this, Where is the hesitation coming from, it is really from your Dads situation, or are you doubtful and undecided about your call anyway? It could be that your fathers situation turns out to be an easy reason to put off and not have to face the implications of a vocation.
So, I would say this: if you really do think you have a vocation, find out more about the seminary you are thinking of, visit them, speak to them, and see if they will accept you. If they will, but it turns that your family really does need you, you can help them but you will know that it is only going to be for a certain time, you keep your eyes focused on your vocation, and ready to follow it as soon as it becomes possible.
God bless.