Concerning the little sister he wanted to say that if there is no blood relation the bond brother-sister is weaker.
What he meant by that is that if they’re not blood related, their relationship can more easily become a romantic one for exemple.
You have less chance to screw your real sister than a step sister (look at Hitomi, Kana, …they are not blood related to their brother).
And for the tsundere it’s true you need to do something to make her dere dere side appears.
So a confession is not a bad idea (even id I have to admit that the way he did it was strange…)