Looking for a yaoi manga where there was a twin brother and sister, the sister was I think supposed to be part of an arranged marriage but tried to elope and the brother was mistaken to be her by a man who was trying to force the marriage, so was imprisoned by him and of course started to fall for him. The truth came to light when the sister and her new husband tried to rescue him. That's about all I remember of the plot

The surrounding details are slightly different, but try "Substitute for Love" (Chapter 6) from Gochisou-sama by CJ Michalski.

Yes, that is actually the one I was looking for, thanks. I guess I kind of misremembered it when giving the description hehe 😁