Estranged Sisters 2000s Women's Fiction

I read this as a paperback which had a marsh or river with reeds on the cover sometime in the 2000s. It could have been published late 90s. Set in the USA, possibly a coastal town. It was very 'women's fiction'. Two teenage sisters were orphaned, they live alone and one sister falls in love with neighbour/friend. Other sister sleeps with him and eventually they marry. The married sister gets cancer and when the other sister goes to visit she sleeps with the husband. It's annoying because it's lingered in the recesses of my brain but Google is getting me nowhere.

Bumping....
Believe it was from the 90s or early to mid 2000s. I think I read it between 2005-2008 but don't think it was a new release.
2 teen sisters.
Orphaned or abandoned.
They didn't go into foster care because older sister was responsible. Neighbours just kept an eye to make sure they survived.
Best friend was a teenage boy.
Younger sister had crush.
One night older sister makes out or sleeps with him.
Younger imsister feels betrayed, older sister and boy get married, and it causes their estrangement.
Many years later older sister is terminally ill.
Wants to reconcile with Younger sister.
Younger sister arrives and things are uncomfortable.
She flirts/kisses/sleeps with sister's husband and dying sister confronts her. Dying sister knows but doesn't seem upset?
Lots of drama and angst.
I read it as a paperback and remember a white cover with a bay or river or some kind of golden watery marsh scene.