Don't want Mom to move in when Dad passes.

Indeed and maybe in the meantime you might get married and then that is an extra excuse for not having room for her to move in. That's not the best reason in the world to get married, but if it did happen, it would add an extra boundary to your having a life of your own! I wish you the very best!

Yup. She probably will want to spend more time with you. She may even want to move in.

Don't invite her. Don't encourage her. In fact, actively discourage her each time the subject comes up.

Visit her where she is. Then you can determine how long to stay.

Take her out. Go to the movies, a live performance, out to eat. Take her back to her place when it is over.

You can still give your mother more of your attention after she becomes a widow, without letting it encroach on your lives in your home.

Tell your mom to stay at her independent living. Set some boundaries about her visits so that you and your sister can have your own lives. Your mom needs to live her own life. Tell your mom no and stick to your decision. She will not like it, but that is her problem.