Disgusted

My sister and I used Mango airlines going home from Port Elizabeth to Johannesburg. We had checked in online and arrived on time at the airport. At the boarding gates I went through and she was told her medium luggage which she was carrying was too big. when she wanted to check in the luggage, they told her it was already closed. We had come to PE on Thursday using British airways and she was carrying the very same bag, which fitted perfectly on the above cabinets without any hassles. However with Mango airlines she was told it's too big. She as a result was left behind and forfeited her ticket. She had to book another ticket with an alternate airline which was in the evening and only got home to her 3 and 5year old children at 23h00 . This is not the first bad incident that we had experienced with this airline. My sister was coming from China and almost missed the flight and was let through because the flight was leaving earlier than scheduled time. She too did not check in her luggage and was let through. When the lady on the check in points saw me coming back without my sister, she called the people inside the airline to tell them that the bag she was carrying was not weighed. At this time my sister was already seated inside and the flight was about to disembark. The flight attendant that spoke to her told her she needs to pay them cash of R1500 or leave the flight, of which she payed it was the last flight during December Holidays. There are several other bad experiences we had with the airline that we had never brought forward which would take me the whole day to list. I would like to find out if the above two incidents are lawful, are they part of the rules and regulations of Mango airlines???