I'm going to ask my 12-year-old daughter to cover herself up more when she's in the house
Is there anything about the way she dresses that bothers you?
Not me personally, but I don't want my guests to look at her in a wrong way.
Fair enough. You wouldn't want that around your guests so they wouldn't objectify her.
I feel like something's not right. What if I am hurting her self-esteem by doing this?
I don't think you will. You can explain the reasons why to her. She should be at that age where she understands.