-Patchwork family: Theoretically the mother and father could have married each other after divorcing with their previous partners and taken their kids with them. That would also leave the possibility for only Haruki and Mitsuki to not be directly related by blood.
-Personally, I don't resemble my parents at all but know strangers that actually do, so just looking alike doesn't prove anything. Mitsuki and Haruki also apparently don't look alike at all. Haruki could be adopted for example while looking like the father or just be a relative of sorts.