Are you filing "married filing jointly"? - then he is, if you file "married filing singely" the money is yours, legally. But ethically, I think it depends on if he is raising your son as his own and contributing to the relationship if he is entitled or not.
File seperatly
You should check with a probate attorney in Florida, but the intestate laws say that you are entitled to one half and his son would get the other half.
Half grandson
Your mother's mother is your grandmother. Her husband is your grandfather or a person unrelated to you who married your grandmother either before or after your mother was born. If her husband is not related to you his son may be your mother's half-brother or may be unrelated. If unrelated the son's son's sister is also unrelated to you. If your mother's half-brother, then the son's son and his sister are your second cousins. If your grandmother's husband is your grandfather, then his son's son (his grandson) and the grandson's sister are both your second cousins.
Someone who stole her half sister's husband?
a step brother If your mom's husband already has a son of his own, that makes it your stepbrother. If your mom and her husband later have a child together, that would be your half brother/sister.
Assuming your son is also your husband's son, then he and your husband's first cousin's child are second cousins. If your son is not also your husband's son, and has not be adopted by your husband, then your son is not related to any of your husband's relatives.
Supplimentary question made into a new question.
It could. Take the son to court before the mortgage co. takes mum to court.
The cast of Half - 2007 includes: Tom Alagna as Husband Laura Alagna as Wife Socrates Ioannidis as Son
Your husband's sister's son is your nephew, just as your sister's son is your nephew.
Your sister's son is not related to your husband's sister's son, because they share no common ancestor.