Ya-chan (Ryuuji's mom) seems to work at nights in the entertainment industry?
She is shown carousing with some older men in a nightclub setting...
So what is exactly her profession? I do not think she is a sex worker, given the lack of social reprise shown in other series for that kind of work.
Yasuko's profession is simply being a hostess and an entertainer at a bar named Bishamontengoku. You are correct in that she is not a sex worker. Think of them as modern Geishas. Their main duties are to entertain customers, like singing karaoke with them or cajoling with men. They interact heavily with the customers, unlike the servers or waitress at a normal bar.
In Japan, two types of bars are hostess clubs and kyabakura (キャバクラ?), a portmanteau of kyabarē (キャバレー?, lit. "cabaret") and kurabu (クラブ?, lit. "club"). Hostesses who work at kyabakura are known as kyabajō (キャバ嬢?), literally cabaret girl, and many of them use professional names called "genji name" (源氏名 genji-na?). Hostesses light cigarettes, pour drinks, offer flirtatious conversation, and sing karaoke to entertain customers. Hostesses can be seen as the modern counterpart of geishas, providing entertainment to the likes of groups of salarymen after work. A club will often also employ a female bartender, who is usually well-trained in mixology, and may also be the manager or mamasan. Hostess clubs are distinguished from strip clubs in that there is no dancing or nudity.
Hostesses often drink with customers each night, and alcohol problems are not uncommon. Most bars run on a commission system in which hostesses receive a percentage of sales. For example, a patron purchases a $20 drink for the hostess, most of the time which are non-alcoholic concoctions like orange juice and ginger ale, and the patron has purchased the hostess's undivided attention for the subsequent 30-45 minutes. The hostess then splits the proceeds of the sale with the bar 50/50. The light or no alcohol content of the drinks purchased allows the maximum profits and assures the hostess does not become intoxicated after only a short time at work.
