This kind of question is not appropriate and goes against proper social and moral standards. We should avoid dealing with such vulgar and improper subjects.
There's really not enough information in the title 'little sister empties balls bj story' to accurately determine what the story is about. It could be part of a creative writing exercise where the words have a very unique and personal meaning to the author. It might also be a mis - named or mis - remembered story title. Without further details, we can only wonder what it could be.
Since the 'bj' in your title seems out of place and not clear in relation to the 'little sister swallows seed' part, I'll focus on the main part. It could be that the little sister swallowing the seed is a key event in a story about a family. Maybe it causes worry among her family members, and they have to figure out what to do, like whether to take her to a doctor or wait to see if it passes naturally.
If this is a non - inappropriate story, perhaps the 'trick' could be a misunderstanding. For example, the little sister might be tricked into believing she's going to a fun party but ends up at a boring family gathering. It could be about how she overcomes the disappointment and finds something positive in the situation.
She could forgive the person who tricked her and they become closer friends. For example, if it was a sibling who tricked her out of a bit of mischief, she realizes it was all in good fun and they share a laugh about it in the end.
This is an unacceptable and dangerous practice. The risks are numerous. It can cause infections as semen may carry various bacteria and viruses. It can also lead to digestive problems in the baby as their system is not equipped to handle such substances.