The cheat sheet version for addressing wedding invitations:
Every “social unit” gets their own invitation, at their own address, and is invited by their own name.
Social unit – a married couple, an engaged couple or a cohabitating couple (and any children under 18 who live with the adult
Own address – where they live/the address that they get their personal mail (& bills!)
Own name – “John Smith” not “and Guest”
So if there is one house with a parent and two adult children you would send three invitations to that house. In the case where you have an “almost 18 year old” living with her sister, it seems that she’s probably entitled to her own invitation, unless there is a reason you think she might not be able to handle the responsibility of responding or something.
For couples who do not live together, it is proper to send each person an invitation at their home – whether the couple is engaged or you just want to make sure your friend has the opportunity to bring her recent beau.
If you want to invite your friend Jane Brown, and you’d like to extend an invitation to a gentleman of her choosing to accompany her you’d call Jane and ask “Hey Jane, I’m putting together my guest list. Is there someone special in your life who you’d like to bring to the wedding?” And when she tells you “yes, his name is John Smith.” Get the proper spelling and address from Jane and mail an invitation to John’s house. If you hardly know John, and you think he might not know who is getting married without an explaination from Jane, you can mail his inviation to Jane’s house – but it should still be his OWN invitation addressed to John Smith, c/o Jane Brown.
If you already know John you might not need to get his address from Jane; but he should still get his own invitation at his house and Jane would get her invitation at her house.
If I understand your post correctly, it seems that every adult you mentioned should get their own invitation – since the couples don’t share an address, and the only ones who do share an address aren’t a “social unit” – they are sisters and a mother & daughter pair. That actually should make the “how do we address them?” part pretty easy – it’s only one name on the envelope! 🙂