I have a cousin who has been recently married. Her husband has a daughter from a previous mate. I have been doing my family geneaology for 10 years now. Is it considered to be impolite or improper to add her to the family tree. I am not too sure about the etiquette when it comes to adding step-children. From what I understand the step-child might not be included because she is not the daughter of my cousin.

Step children are not shown as children of that person, most faimly tree show who they marry, so this child can be shown as a child to the wife, though showing a prior husband for the wife.

Show her as his child and if you want more completion you can include the child's m

Show her as his child and if you want more completion you can include the child's m
Sorry... to continue... the child's mother in parenthesis with marriage and divorce dates. It depends on your purposes. For historical purposes all the connections could be of interest to future generations. To hand on the wall now, less detail is more sensitive but the child should be included no matter what.