I believe it's only with cousins and they are side characters. It's been a while since I read that, but I believe that's why the tag is there ... which is cousins really considered incest? Isn't it legal to marry your cousin in Japan? So would it still be considered incest? but that's off topic, sorry.

A biological brother falls in love with his long lost sister, but nothing more than a kiss happens, and he's not the main lead she falls in love with.

Yeah, AFAIK in Japan it's legal. Maybe why CousinxCousin is considered incest here is because in many countries you can't marry your cousin? Well, tho in my country it's also legal..

Much like Fruits Basket the incest here is a case of cousin incest (CielxElizabeth), doesn't go far- pretty one-sided, but when you keep in mind the setting of the series it is sort of commonplace.

Omi, one of the main characters finds out his lover, Ouka, is his half-sister. From what I remember they decide to just be siblings after the big reveal and the relationship is done for. They didn't go far into the relationship and later on Omi learns that they were actually cousins. Twisty incest~! So still incest, but legal incest! :D

The character January is a child of incest between brother and sister. The brother and sister were raised separately and were married to each other in order to preserve the royal bloodline.This is told as a backstory.

The MC's sister had incestual feelings for him. She kissed him once but nothing came of it. Another girl and her brother were attracted to each other but did not enter into a relationship. There is an uncle who takes sexual photos of his niece. Also, there is some incest subtext between the MC's childhood friend and her father. There are scenes where they are in the bath together.

I've only seen the movies, so for all I know there may more to it then that in the novels, but Mikiya has a teenage sister who is obsessed with him. Nothing much comes of it, however, that could be different here.

At the beginning of the story the mc & this guy fall in love even though they are "half siblings." After the mc goes to the other world you don't really see that guy again until the end so it isn't as important. It turns out though that they are not half siblings as you reach towards the end of the story since the mc was never from the world she grew up in and she was born in the other world. Everyone from the modern world just made it seem like they were half siblings since they didn't doubt that the woman that found her was her real mom when she wasn't. It also didn't help that that mother never explained anything to her.

The mc and her love interest are cousins

The two twins are in love with each other

I don't exactly remember everything but I know that the three main characters which are Ryuto, his nephew, and the mc Shuka are all related. I think it was that the Ryuto and shuka have the same grandfather.

Is their love considered sin in their world?

I don't remember anyone caring, of course I could be wrong. The world was in a bit of chaos and people kept trying to capture the two mcs in order to use their powers to their own gain. In addition the siblings get separated and it takes some time until you see them together again.
There were just so many other things going on that I don't think characters in the manga were preoccupied with whether or not the mcs love was a sin or not. I don't recall if the story actually mentioned that incest was a sin or not.

The twins exchange kisses and hugs from time to time, but these are signs of a close brotherly affection and they become scarce as the brothers grow distant later in the story. Kisses are basically meant to put on public display the siblings' strong emotional dependence on each other.