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.