Refused my sisters over 18 entry

I went to this Casino with my sisters whom are of age.....one is 19 and the other is 21...the security guard asked them for identification even though they are over 18 as they do not allow over 18 but under 21 entry without identification which we didnt know and only found out from the security guard expecting me to drive all the way back home to fetch identification. I asked for the manager because there wasnt any signage anywhere that says so otherwise i wouldnt have bothered bringing them along. Mr Daniel Msimango who is apparently the site manager came up and I related my query with him.....i felt its unfair that there isnt any signage that says so or even on their website its not put up. He left me while i was talking to him which was very arrogant and rude for a manager. I feel even if its their rule it should be put up so that we bring identification in the future. He is very rude and should buckle up his customer relations.