My sister Maham F23 has a sugar daddy

me and my sister are living in UK for studies, but we have been short on money since we arrived barely making our living, past week was amazing my sister got a job and she brought pretty good money in so-called advance, We ate well slept well at-least i did. I didn't really ask her what her job was, today when i came home, there was this big bulky dude standing at my sisters door and i could hear thumping sounds, sighs, clapping , spanking and some other nasty sounds. I kinda knew what was happening but i just couldn't believe i tried to get in but that guard barely moved when i tried, he told me to go sit there on couch till its done. then after 30 mins later i saw this fat black motherfucker walking out. Looked straight into my eyes without any disgust and said to me hey wtsap. and then just walked out. I gave my sis 5 mins to properly cover herself then i walked in, she was barely holding up. I told her to stop, she's saying she's okay. and i dont need to worry.
so is this normal in sugar daddy situations? what should i do?