So this manga is ending soon I gather? With all of this sudden drama happening at once in an otherwise lighthearted manga it usually means the rushed ending is near.
I'm also betting we're getting a revelation that they're not actually blood related brother and sister now that the parents find out, maybe some son/daughter of a cousin or something, so this relationship is actually sorta legit, because Japanese love this trope where it's your sister but not really so it's a-ok,.