#riptidethepen

Within the context of characters we talk about, everyone has both a bloodline and a sect, and they’re sometimes the same big group. Everyone with a Lan surname is understood to be both part of the general Lan Sect (which trains people in a set of principles and cultivation techniques) and part of the Lan Clan bloodline (so they have the fancy forehead ribbons instead of the plain ones). This is one reason why it’s such a big deal that Sizhui carries the Lan surname: he’s not actually of the bloodline, but that lie was used as another layer of camouflage to keep him safe. It’s also part of Lan Xichen and Lan Wangji’s discomfort with Su She. He was part of the Lan Sect, because they were training him in their specific cultivation path, but then he took that training and set up a rival Sect that very obviously uses that training. The Lans, at least, address this as “inner” disciples (clan) and “outer” disciples (sect only). All disciples are part of the overarching Sect. Wei Wuxian’s particular situation is complicated because he would be part of the Jiang Sect, but Jiang Cheng and Jiang Yanli treat him as family even though he’s not blood-related. He considers them siblings, but also has a connection to either the Clan or Sect (not sure which? it’s a bit of a special case) of Baoshan Sanren through his mother (thus the “we’re the same age but you’re my uncle” conversation with Xiao Xingchen in the drama).

As another example, Meng Yao has been part of multiple Sects (Nie, Wen, Jin), but it is only when he’s officially acknowledged and renamed by Jin Guangshan that he becomes part of a Clan. Mianmian leaves the Jin Sect and that choice is treated as not very important by Jin Guangshan partially because she’s a Sect disciple, but not part of the Clan, and probably also partially because she’s a woman and disagreeing with him. (Jin Guangshan is distinctly unconcerned about his bloodline, given all the bastard children he doesn’t acknowledge, but politically it would be more significant if someone carrying the Jin surname left).  Additionally, I’m pretty sure marriage politics are also at play: Jiang Yanli does not (quite) become Jin Yanli when she marries; she’s still part of the Jiang Clan and the marriage is an alliance of bloodlines as well as Sects (I’m basing this on the conversation she, Wei Wuxian, and Jiang Cheng have in Yiling before her wedding, but I could be missing something). She has joined the Jin Sect as well as her husband’s family, and her child is part of that Clan.

TL;DR, an individual can potentially join or leave or establish a Sect, but Clan is genetic. There are probably ways for people to be expelled from a Clan, but the novel doesn’t really explore that.