Is it considered a mistress when you have a relationship with a person who already has a family but they are not married?

It is considered a "mistress/lover" when you're yourself in an exclusive relationship already (whatever its status is) and have an other sexual / love relationship with someone else on the side. You are that person's lover/mistress, if she's at the moment in a relationship with someone. It doesn't matter if they're married or not, it's still "cheating" when people aren't married :/ The only situation where it wouldn't be called like that, would be if you are (or they are) in a polyamorous relationship, which means both persons in the relationship are ok and consent freely to have other romantic and/or sexual partners outside of the relationship (polyamorous meaning, roughly : "being able to be in love with different people at the same time and acting on it openly with mutual consent").

Hi! It sounds like you are experiencing some undecidedness and confusion. These types of boundaries are always quite blurry. I would encourage you to think deeply about the terms of your relationship and talk to your partner! Is it exclusive on your end? What type of relationship does your partner have with their family? Have you had a conversation about exclusivity? Is exclusivity important to all parties? Secondly, I would like to know what the word "mistress" means to you? What importance does that label have for you, for your community, for the people involved in these relationships? I hope this was helpful! Please know there is a whole community of people here to listen and do their best to help you!

The correct definition of mistress is: " A woman (other than the man's wife) having a sexual relationship with a married man". This woman might not be married, but since she is involved in a relationship with someone, many people would use the term for the situation. Regardless of the title we give the person, the actions are the same. Having a relationship with someone that is already in a relationship with someone else, despite if she is married or just dating, it's considered cheating. And it will cause pain for the people involved even if they are not married.

I have a family, albeit a small one. I also have a girlfriend. Neither I nor any of my friends would consider her my mistress, they are all happy we are together. In my case, my wife passed away over 8 years ago, and there has been a great hole in my life. I believe that it depends on each situation as to whether or not a woman would be considered a mistress. If my girlfriend moved in with us, and I took up with yet another woman, I think she would be my mistress. It all depends on the situation.