ABANDONED. Book about sisters who find each other, one was actually the delusion of the other?

[Bess]

I think this book was written alternating between the perspectives of the two sisters, but the focus was definitely on one. She was an adult, I believe the book was set in America, and part of the book was about her being pregnant/having a baby, she was struggling and very angry. I think the baby was an accident from a one-night-stand or abusive relationship, and that something happens to it. She found out she had a sister she either didn't know about or thought was dead (on a website for mums-to-be? there was definitely such a website in the book), they write to each other then meet at some strange grand mansion in France where the other sister lives, but if I remember correctly she ends up just being a delusion. The narrator was unreliable and kind of a sinister character. Quite a uncomfortable/disturbing book. Some details I mentioned here might be off or fabricated but I don't think that's the case. Thanks in advance for any help!

Edit: I read this book two years ago, I'd say it was surely published this century. I read a physical copy. It's also genre fiction. It started out contemporary but moved into being a thriller. It was set in the present. Cover art may have been photographic. It was aimed at adults.

[Lobstergirl, au gratin]

When did you read it, or about when was it published?

Is it genre fiction or literary fiction? If genre fiction, what genre? Romance, horror, etc. Is it set in the present?

Cover art?

[Bess]

I'll edit my original post with my answers, thanks for the suggestions! I'm new to this.

[Kris]

Bess, is the baby with her when she goes to France?

Was is definitely France - and if so, was it a large city like Paris or the countryside?

[Bess]

I couldn't say. It was the French countryside.

[Lobstergirl, au gratin]

Bess, are you still looking for this or did you find it?

[Lobstergirl, au gratin]

Bess left the group. Moving to Abandoned.