So I just did this exact same thing! I asked my sister if she wanted to do x activity with me this weekend. We settled on the day & time that we both could do it. Then I said, "should we invite anyone else?" And we agreed to invite our friend "Mary". I emailed Mary and said, "Sis & I are doing X, Saturday at 1, do you want to come?"
Mary wasn't B list. The activity is something I specifically wanted to do with my sister so it was important to pick a time that suited her. We thought about asking Mary along, not as a B list person, but because once we'd hashed out the details, we thought it would be fun to have a third person along.
Mary wasn't put off that we'd already picked a date and time. She was happy to be invited and since she's available, she's coming.