I think the father is his sister's(brother's) ex
Well yeah, probably. And he might be the reason why the brother changed gender. But i feel like we have been getting hints about some stuff but everything is confusing...
Maybe they are siblings from different mothers since the food twin did say brother
No i think he said brother because thats how you talk to older men (and if a lot older, then it would be uncle). For example, i think the MC's best friend also calls him/her "brother" or "sister"
Ok so from what i understand. The father of the twins did something to the big brother (probably related to him changing gender). And now for some reason, the "good" twin thinks that something bad could happen if Mad Dog dates the MC?? As if he is meant to become a bad person like his dad? (I feel like the "good" twin even mentionned that Mad Dog is also responsible for something about the bro???)