Inviting themselves
I'm so baffled by this, I don't even know where to start.
My fiancé received a phone call yesterday from his dad's girlfriend. He was at work, so he was unable to answer it, but she left a voicemail. The voicemail asked him when she and her daughter would be receiving invitations to our wedding. Our wedding is not until March of next year. I haven't even ordered the invitations yet. Save the dates went out back in August. She was not included on the list because she wasn't dating my fiancé's dad at the time.
Just some background information. They were dating last year and broke up right after Christmas. They got back together in September. We got engaged in February. When my fiancé and I started working on the guest list, I asked his dad about inviting her. (they have been on again, off again before) He said he didn't care if she came or not. My thought process was his dad is getting a plus one, so if they get back together she will come with him. His dad is a ladies man. We joke about it with him all the time. Our fear with inviting her to the wedding was that they may break up again and he will invite another lady. So we told his dad that he has a plus one and he can choose to bring her or someone else of his choosing. It doesn't matter to us. We want him to be happy and comfortable at the wedding.
I have read a ton of discussions and my fiancé knows he has to respond to her. 
(Thank you fellow knotties) My concern is, if his dad tells us she is his plus one, do I have an obligation to invite her daughter and significant other? Her daughter is around our age and does not live with her.
I will also add that my fiancé and I are the only ones paying for the wedding.
Thanks for the help
Re: Inviting themselves
I think you are smart to not send this woman an invitation of her own. The only person who can determine if this person is your FFIL’s SO is your FFIL. I think right before your invitations go out your fiancé should ask his father if FFIL wants this person’s name addressed on his invitation or if he would rather an un-named pulse one.
For the current phone message, I think you are far enough out to just put her off with something along the lines of, “Oh we have not even ordered invitations yet, when the time comes check with FFIL”. If she keeps asking as time gets closer you should keep redirecting her back to FFIL, he is the one who will eventually have to tell her, “Yes you are my girlfriend and invited” or “No you are not my date”. That is not your job or responsibility.
As for the grown daughter, no you don’t have to invite her.
You do not need to invite her daughter. It was wrong of her to assume that you would.
@downtondiva She will definitely be invited to the wedding as FFIL plus one if he picks her. It just don’t think it’s neccessary to give her an invitation addressed to her only.
@MobKaz Funny you mention her having the cellphone number. I was just thinking how she even got them. She has mine as well. I can’t recall ever giving her my number and I know my fiancé didn’t give her ours. It must have been FFIL.
But you've handled this well so far.
O.k. how FI should respond is "Invitations have not even been ordered yet.. Those who are invited will be receiving their invitations approximately 6-8 weeks prior". And yes, use those terms because she may or may not be invited given the relationship at this point. If she is "on again" at that point of the invitations being sent, it'd be diplomatic to invite the daughter...