This is my take on everything so far: I honestly feel like with the little drops they had in the previous chapter and this one, Sera may have tried to be an idol but failed to get into an agency after multiple attempts. This led her to go to the maid cafe to get more experience or to deal with the trauma of being rejected. Kira succeeding in being an idol and being better at everything Sera tries to do is why she doesn't talk about her being related to an idol. Sera doesn't want to explicitly use Kira's popularity to gain fame which is why she always tells her to not do anything crazy after she complains to her.
Just my opinion, but I think that when you're a nobody, an Idol who got rejected and starting from zero, there's only 2 options for you (aside from giving up): Being a street artist, trying to win some public atention and fannbase or work on a cafe/maid cafe. These are places where some Idols can being their carers, amass a loyal fanbase and hope to meet with a producer who happened to be there (plus, money, we all need money). And, as we know since ch 1, Sera did never sucess on any of these things, making things worse for her mental healt-