I've read this story months ago? but regarding the whole sibling situation with the main characters I would say it's something like this altho not the most specific but you get the ji**:
so in the PRESENT TIME from my understanding, they are indeed twin siblings. Born from the same mother etc.
BUT in the PAST TIME, they were not biologically related, although they did have a borderline toxic love/hate relationship and the PAST FL also had a twin.
Also, reincarnation happened to both of them, I think the FL has a new soul in her PRESENT body while the ML got 2 clashing souls in his PRESENT body.
(so basically his father is an a-hole whom he betrayed by giving their fam recipe to the PAST FL, then daddy got mad and experimented his soul to be put in the new PRESENT TWIN BODY, which already had a fixed soul).
so as the NEW ML grows up, the two souls in the same body is clashing in every aspect
in conclusion:
The FL had a brother in the PAST and in the PRESENT.
The FL's soul is different in the PAST and in the PRESENT.
The ML was once someone FL knew in the past although not related biologically.
Now, in the PRESENT, they are related biologically.
*correct me if I'm wrong, I'm just recalling what I think I know when I read the story many moons ago.
*I'm basing these content in the manhwa