Maybe you need to learn some reading comprehension skills.
None of what you said offers absolutely any logical reason why she would offer it in the first place (when we know she won't take advantage of it). Why she wouldn't take advantage is fecking irelevent (in the course of this question...though there is also no logical reason for that either).
Look, let me spell out the writing for you.
1. She is too busy for her fiancé ? Check.
2. Too busy for anyone else? Check.
3. Relationship falling apart? Check.
4. Won't/refuses to cheat? (even though sis talks her into...but the dev says she won't lol) Check.
5. Is lonely? Check.
6. Solution? Offer fiancé open relationship? Check. Reason? Wtf knows...
7. Profit?
Now, there are very, very few reasons why a woman would ever offer her partner this relationsip. In fact, there's only 3. Gets off on it (nope...not in this case)...wants to fuck others herself (nope...neither here either)...or wants to offer it her partner to blame him for the breakup she wants (possibe but not as far as the dialogue goes at least).
I'll await an aswer...a logical one...reasonable....any at all....just one.
1. Agreed.
2. Not so sure, she finds time to meet her sister for drinks.
3. Not really falling apart, life is just getting in the way of the relationship. Both still love each other.
4. You know that the Devs didn't tell her that, right? She suggested the open relationship intending to get the benefits. It doesn't mean she will get them. Unless I missed a bit of dialogue where she says she can predict the future, she doesn't know what will happen.
5. Yup, well, she has her sister now and again.
6. The reason, for both of them to explore their options. He jumps right into bed with Vicky a couple of hours later, she takes her time. She doesn't know that she won't find anyone at time of offering.
7. We will see.
Your three reasons. 1. Yup, not going to happen. 2. She intends to play the field when she offered it. She doesn't know the devs aren't going to let her. I intended to do a lot of things this year, doesn't mean I have been able to do them thanks to this bloody pandemic. 3. Again, not going to happen. They both still love each other.
One last time, to make it easy for you to delete most of what I write when you quote me again: She offers the open relationship fully intending to also take advantage of it, but she cannot predict the future. Just because she intends to do something, doesn't mean she will get the opportunity. She isn't one of those characters like Deadpool who can break the 4th wall and know what the devs have in store for her.