I had missed the fact that this was disability SS. Did your sister just begin SSD recently? There is a trial period of nine months, not necessarily consecutive, in which the person may work without losing their SSD.
After that period, the limit you speak of is presently $1,000 per month. At $250 per week paid by week for 52 weeks, she would be over this limit by $1,000 for the year.
Yet, how do you know that she gets this amount each and every week? She could stop working near the end of the year. The $80 cash could have been reimbursement for business related expenses that had been incurred by your sister.
There are way too many scenarios. You are simply jumping to a conclusion due to your apparent dislike for your sister.