Definitely not appropriate. Family relationships are based on love, respect, and appropriate boundaries. This kind of description involves extremely abnormal and unethical behavior. It is not something that should be part of any story or real - life situation. We should uphold positive family values and ethical standards.
First, if it's just a stupid joke among peers, you can have a conversation with those involved about appropriate language and respect. Tell them it's not okay to use such vulgar terms in relation to your son.
Well, if not in a vulgar sense, 'huge cock' could potentially be a misnomer or a very strange way of referring to something large. Maybe it's a big rooster on the family farm that the stepmom has some sort of story related to. But this is a very odd and uncommonly used way to phrase it if not being inappropriate.
A 'huge tit wife' story could be about a wife who has a very large heart, symbolically represented as 'huge tit' which means she is extremely kind and loving. For example, she might be always there to support her family, like taking in stray animals and nursing them back to health, and being a pillar of strength in the community by volunteering at local charities.
I'm not sure specifically which 'huge cock story' you are referring to. There could be many stories that might use this phrase in a non - vulgar way, like a story about a large rooster perhaps.