That's true LXC might be the older twin but he might be older in general because you wouldn't call your older twin 'older brother' you'd just call them by name like how JC calls WWX by his name. There's also the fact that Jiang Cheng and Wei Ying called themselves the twin pride of Yunmeng even though they weren't twins so being called the twin jades doesn't mean that they are actually twins.
Well his father was the sect leader so he needed heirs right? So I'm guessing conjugal visits? Also I think they had to consumate the marriage to make it absolutely binding. So like a sect heir and a spare in case something were to happen to the sect heir. I don't like the use of the word spare for children but in this case it made sense.
you know, Ive read the novel, watched the drama and anime, read this webtoon, all of them several times but ive yet to understand how lan zhan and xichen were born when their parents locked themselves up so they dont see each other. and considering age difference between lans , im sure those parents have met more than once