Fresh seafood shouldn't be overly "fishy". It's generally older seafood that will get a stronger "fishy" smell and flavour. There are 4 approaches to a "less fishy" result:

1. Clean the fish properly from the initial processing step

Remove organs, scales, and pay special attention to cleaning blood and cloudy white membrane inside the fish’s belly, as these are the causes of fishy odour affecting the food quality. For certain fish types, like snakehead, carp, and tuna, check the fish bones. There is a white strip on both sides of the bones that causes the fishy odour. When processing, cut close to the strip to completely remove it, making the fish more aromatic. For freshwater fish such as catfish, tilapia, and eel, use stove ashes to rub on the fish skin to remove the slimy substance. If there are no stove ashes, you can pour hot water on the fish and then use a knife to scrape it off. This method not only removes the slimy substance but also effectively eliminates the fishy odour. 

2. 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. The milk will help to neutralize the strong fishy taste of the fish.

3. Disguise fishiness:

The other angle is to "cover up" the flavour with lots of spicy glaze or garlic/lemon flavours instead. Cook the fish in a way that will not allow the strong fishy taste to come through. This can be done by cooking the fish in foil or by adding other ingredients to the recipe that will help to mask the strong fishy taste of the fish.

After cleaning the fish, you can use diluted white wine to marinate the fish for about 5-7 minutes, then dry it. This is explained by the reaction of white wine with free acids in the fish, forming a fragrant ester. Additionally, when steaming fish dishes, you can add a little white wine to the water for a similar effect. For seasoned fish, adding a little white wine is also a good option. It not only removes the fishy odour but also makes the fish tender and more aromatic. Using spices with distinctive aromas such as pepper, garlic, chili, ginger, and cilantro to cover up and eliminate the fishy odour. This method not only helps prevent fishiness but also adds a delicious flavour to your dish.

4. 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.