Indeed, Wolverine and Sabertooth are considered brothers in the comics. However, their interactions are filled with rivalry and hostility, making their brotherhood a very strained one.
A common theme is their rivalry. It's often explored in depth, like in 'The Eternal Rivalry' fanfic where their battles are not just physical but also a battle of wills. They constantly try to outdo each other.
One great Sabertooth vs Wolverine fanfiction is 'Clash of the Mutant Titans' on FanFiction.net. It really delves into their complex relationship, exploring their past battles and grudges. The author does a wonderful job of writing their intense fight scenes and the underlying emotions that fuel their rivalry.
In 'Wolverine Origins', Logan's wife was Kayla Silverfox. Her story is one of love and tragedy. She was a mutant with the ability to manipulate others' minds. She was sent by Stryker to get close to Logan. However, she truly fell in love with him. Eventually, she was killed by Stryker as part of his plan to manipulate Logan further, which was a great loss for Logan and a turning point in his character development.
Well, in some fanfictions, Natsu might initially be seen as an outsider by Sabertooth and face hostility. But as the story progresses, he wins them over with his strength and determination.