Let’s say you married a widow, and the widow had a daughter. The widow’s daughter married your dad. That makes your daughter your mother as she is married to your dad.
Now if you had a kid, that kid is a brother in law to your dad, which would be your uncle and also the brother of your step daughter, who is your stepmother.
Ok so now your stepdaughter has a kid, which would be your grandson. This all makes your wife your mothers mother also, or your grandmother. If your wife is your grandmother then you are her grandson. As your grandmothers husband it makes you your own grandpa.