Is there sin in lying and employing tricks against one's brother-in-law, and seizing his balance? Or does the sister bear all the sins? And is it obligatory to seek his forgiveness?

You must return the money you took from your sister's husband because it was a loan. Your sister must repent to Allah Almighty for revealing her husband's secret and checking his bank balance, which is forbidden if he kept it hidden and did not wish for it to be disclosed. You both must also repent for having access to the man's secret. Your sister should seek forgiveness from her husband without informing him of what happened to avoid conflict. You are not required to tell him about your lie; sincere repentance to Allah Almighty is sufficient.