How does one remove the "fishy flavor" from seafood?
Fresh seafood shouldn't be overly "fishy".  It's generally older seafood that will get a stronger "fishy" smell and flavour.

That said, there are 3 approaches to a "less fishy" result:


Absorb/reduce fishiness:  you can always try something like soaking it in milk (which you can then save to use for a fishy bechamel when your sister isn't dining with you).  You could then rinse it off and cook it however you want.  This should help "absorb" some of the fishiness and make it milder. 
Disguise fishiness: The other angle is to "cover up" the flavour with lots of spicy glaze or garlic/lemon flavours instead. 
Pick mild seafood: The other thing to try is to pick a more mildly flavoured fish (e.g. a white fish instead of something like tuna or salmon).  Avoid oily fish as they tend to have a stronger flavour.  Choosing something really fresh also falls into this category.