Were Luke and Leia always intended to be brother and sister? It's a question that has plagued us for decades.
A refresher: in the original movie, the two meet. There is no hint of a pre-existing relationship. In fact, there is just enough of a hint of a budding romance.
In Empire, Leia kisses Luke, and he is rather happy about it. Now, she clearly prefers Han, and the kiss is mainly aimed to piss Han off, but still. Oh, and Yoda mentions that "there is another".
In Return, Obi-Wan reveals to Luke that Leia is his sister. Luke reveals it to Leia. Leia: "Somehow, I've always known." Gross.
So, did George Lucas always intend for them to siblings, or did he retrofit it to the narrative? The problem is, we really don't know when a lot of the plotting was done. Lucas has famously spread the idea that he basically had all these movies planned from the start. Supposedly, he knew what he was doing from the beginning, even the prequels. This isn't true, but that's the official line.
Regardless of what Lucas claims, we're willing to bet that it was not originally planned. Do not forget "there is another", as it leaves so much open. Lucas gave himself narrative flexibility: "another" can be anyone. By the time Return came around, it made sense for it to be Leia.
Also, considering they were cast together, Mark Hamill is just too goyishe to be Carrie Fisher's brother. This becomes incredibly evident in Last Jedi: it is really hard to imagine a universe where those two, who look nothing like each other, are twins.
See, we can reduce many questions to Jew or Not Jew!