How to cook squid to get rid of the ‘urine’ flavor? What causes that flavor

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:

1. 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.
2. Disguise fishiness: The other angle is to "cover up" the flavour with lots of spicy glaze or garlic/lemon flavours instead.
3. 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.

I think it's a fine question. If I were in your position, I'd definitely go for option 2 - I think pre-cooked and re-steamed crab would end up a lot like oversteamed crab: mushy and gross.

As for keeping them alive, when I was a kid we used to go crabbing every weekend. Mom would just keep them in a cardboard box in the kitchen until we were ready to eat them for dinner (much to my dad's dismay).

According to Bluecrab.info:

Crabs can live for several days out of water as long as they are healthy and are kept cool, moist, and stress-free. The optimal temperature is about 50° F., anything colder will ultimately kill them.

If I remember correctly, mom would keep a damp towel on top of the box. But that may have been dad not wanting to look at them.