@yannickq in this case i'd actually say it's biological!
family relationship terms are a bit of a mess to translate because there's a lot of overlap, haha. usually, we refer to relatives in the same generation as us as 'sister' or 'brother.' if we want to get more specific about exactly how they're related to us, we can prefix it as with something that indicates that. so yu nian normally refers to yu qing as just 姐 (older sister), but when he's explaining his relationship with her to meng yuan, he calls her 表姐 (cousin with a different last name + older sister).
but at the same time, people will use family relationship terms to describe... well, pretty much anyone. it's pretty common for people to refer to their friends as brother or sister, for example, but it goes beyond that, too. some people like to call their boyfriends 哥哥 (big brother), and some people might call a hot celebrity they like 小哥哥 (little big brother), and so on and so forth. so it's ... really hard to figure out what someone's relationship is just from those terms.
yu nian tends to speak pretty respectfully, so he uses familial terms for most of the people he speaks to. in chapter 38 he called an older family friend 古爷爷 (grandpa gu) and he calls meng yuan 孟哥 (brother meng) all the time, but that doesn't mean they're actually related. so people probably thought he called yu qing his sister because that's just how he talks. no wonder they're confused!
when he clarifies this point to xie you in his last text message, he uses the term 亲姐 (blood-related sister). again, 'sister' here isn't used the western way (i.e. from the same parents), but it means that they're blood-related relatives of the same generation.
whew! that was long (and i actually learned a few things researching this) but i hope that answers your question, haha. family relationships are really annoying to translate into english because there's a whole different set of information encoded in the terms in both languages, so some things get dropped by necessity.