My memory of this manga is vague, but I remember the main character was female. Her fiance (or maybe husband? I think he was a prince) was cheating on her with his sister (yes, HIS sister)

I think he had a spear that belonged too or had the spirit of this dark yandereish goddess in it?? The FL had a similar weapon, I think.

Pretty sure you're talking about The Do-Over Damsel Conquers the Dragon Emperor (unlicensed title is Win Over the Dragon Emperor This Time Around, Noble Girl!).

Just be warned that though there's a really creepy age gap thing going on with the 10 year old FL and 19 year old ML. They somehow managed to make the FL's cheating ex's incest only the second creepiest part of the story 😬

Yeah, that's the one! And, now I remember why I dropped it 😅 Thanks for solving that for me!

No problem!