It's like STDs - if you're careless and go sticking your junk everywhere without taking precautions, you'll probably catch something cruel, eventually.
That's why I run Linux. Running Linux pretty much rules out any possibility of having sex, and hence any chance of contracting an STD.
I can't speak for your sister and her child's reason for excluding sea creatures from their vegetarianism, but for me pescatarianism is a stepping stone to vegetarianism. I wouldn't be able to become a full vegetarian by going cold turkey
Feel free to call me a "giant hypocrite" or whatever you want, but for me becoming a vegetarian is a major lifestyle change that has significant psychological, cultural, and dietary challenges and it isn't going to happen without approaching the problem intelligently.
Nothing will ever be attempted if all possible objections must be first overcome. -- Dr. Johnson