This is the old maxima "Divide and conquer" - make two sister nations fight with each other for your own benefits. Then you can always look innocent and blame one or the two fighting parties for their own fight.
And it's also a very convenient propaganda technique used by Russia all the time - "look we didn't really want to do this, it's the foreign interference in Ukrainian politics that made us attack them!". Like, ok, sure - but you are still the ones pulling the trigger.
Ukraine is not sister nation to Russia. This is old Russian propaganda, in reality, Ukraine is old European nation and Russia is Asian (just remember about mummy at center of Russian capital, and think, where at Western country exists something comparable).
Claiming Ukraine sister nation for Russia is not exact same, but similar to claim Israel sister nation to Iran, or Japan sister nation to China.
