The option still wasn't there so I forced them to be married, I think the issue is she's connected to her family in Sunset Valley, including Erik. But I forced them to be married with a mod.
Edit: This Agnes is different that the starting one but she always is connected to her sister and parents, when I put her in the same town. She's also connected to Erik from the start, so it might be I have to disconnect them, then try to marry them again.