As per DWMB2, we need a lot more info to help you, incl PCN, all sides, and a GSV link to the exact location. Do not, please, redact car reg, or PCN number, because we can look to see the CEO's photos on the council website.
However, from your narrative, it would appear your sister was parked waiting to board your MIL, correct ? The more you can tell us the better.