[m] It's about these two twins ones a boy the other a girl and the girl is being made to marry this rich guy but she has someone she loves already. So as the brother is leaving his sister's place after hearing that the two will elope before the marriage but the bro gets kidnapped by the person the sister was suppose to marries bro. The guy ends up thinking that the bro is the sis since they are twins but he ends up falling in love with the boy and doesn't want to give him to his brother.
I know it's probably only a oneshot but if anyone has the slightest idea of what it may be it would really help, I've been racking my brain over this for quite some time now

Is it Kimi wo Metoraba?

it sounds similar to moshi mikawari no koi demo - 6th chapter of Gochisousama by CJ Michalski, but in this one it's not the girl who is forced to marry someone but her boyfriend with whom she plans to elope a day before his arranged marriage. The twin brother gets kidnapped by the brother of his sisters' boyfriend. The rest of plot is as you described it.

Oh that's it thanks so much mizerykordia I feel really stupid I should have know it was from CJ Michalski but I hadn't read it in a while and could only remember the plot but thanks again >.<