Is it ethical to ask the postdoc employer to cover my trips for visa purposes as well as my main travel fees? I will soon start doing a postdoc in country A, and the employer says there will be a generous relocation package. My home is in country B and I did my PhD in country C.

The question is: in order to apply for the entry visa for country A, I had to first return to my home country B and apply from there, instead of applying in C, as my PhD visa for country C ran out. So the relocation involves two travels, from C to B, and then from B to A.

Would it be ethical to ask the employer to cover my flight from C to B as well? Yes, the primary reason for coming back to country B is visa application, but this can also be considered visiting family and therefore not related to work. Any suggestions?
In the cases the I have known in the U.S., the relocation allowance is a fixed amount of money, and the university is fine with it being spent on anything reasonably related to relocation.  If this is the case with your university as well, then they'll probably be fine with it being spent on your combination of expenses---but if they combination of flights is expensive, you are likely to hit the cap and have some of the costs become your own responsibility.

The exact rules and regulations, however, can vary highly from place to place, so you really need to ask your university.