Yes. For instance, a story about a sister getting into her first dance competition and the mom being there to help her practice every day, sew her costume, and be her biggest cheerleader on the day of the competition. It shows the love and support within a family.
Being disrespectful is another reason. If you talk back rudely to your sister or don't listen to your mom, you're likely to get punished. Say you roll your eyes at your sister when she asks you to do something simple like pass the salt, that's disrespectful. Your mom might ground you and your sister might stop sharing her things with you.
Perhaps they were boating. Sister and mom rowed the boat gently on the lake. The sun was shining, and the water was sparkling. They were chatting and laughing, taking in the fresh air and the wonderful view of the lake and its surroundings. It was a lovely experience for both of them.
There might have been an occasion where they planned a surprise party for a family member. Mom took care of the food while sister decorated the place. They worked so well together and the party was a huge success.
Another way is to participate in some activities together, like going for a walk in the park or watching a movie at home. This gives you more opportunities to communicate and understand each other better.
It's not okay at all. In most societies, there are clear norms about appropriate relationships. This kind of situation goes against the trust and respect that should exist in a relationship with a girlfriend, and it can lead to a breakdown of multiple relationships and a lot of emotional pain.
Sister sex fanfiction involves inappropriate and often unethical content that violates moral and family norms, so it is not something that should be promoted or explored in detail.
If the sister catches mom making up or 'screwing' stories, she could first have a private conversation with her mom. Let her mom know that she noticed and that honesty is important in the family.