My parents are asked to ring the downstairs doorbell and than come on in! I don't have time to run up the stairs and open the door for them and I can't always do that either (when I'm busy attending to the children for example) My husband is home during the day but even when he isn't, it doesn't scare me to have the door open. I ask them to ring the doorbell so that it gives me a heads up in case I'm in the washroom or something but it also allows me to know that someone is in the house. If someone were to ring the doorbell and than I heard them walking around upstairs I would know it's not a parent and I would take the steps to assure we are safe.

If you prefer to answer the door though, tell her now before it goes on too long and becomes kinda awkward would be my advice.