It depends. If it’s a temporary stay, in between having your own home, then yes, it’s like being homeless. It’s not the same homeless as actually living on the streets, but you don’t have you’re own home, and you feel like a guest just waiting to finally get your own place.
When I lived with my inlaws for a few months I had 95% of my belongings in storage. It was practically like living in a hotel, except you have family around, which is nice in one way, but difficult in another.
The term homeless is commonly used to mean someone who can’t afford or keep a home, and has no one to stay with, but not having your own home feels “homeless.”
Completely different is when a relative moves in, and it is considered permanent, or a home that is truly shared. I have friends who have a parent or parents living with them, and the parent isn’t homeless living there, because they are fully part of the household, and the intention in everyones mind is that they will be staying there long term.