That is because the legal definition of genocide requires two layers of intent. First you have to do acts of genocide intentionally, like stealing children. Second for it to be a genocide it has to be proven that the acts of genocide was done with the goal of destroying a national group in whole or in part.

So everyone agrees that russia are commiting acts of genocide but it's still not proven that this is their goal.

And no it's not a legal obligation for anyone to do anything unless the UN security Council decides to do something. And Russia has a veto there.

There is currently a case in the international court of justice about Russian claims of genocide in Ukraine. So far the court has decided that russia is legally obliged to withdraw untill its clear that ukraine is commiting genocide (they are not), but the fear in the west is that ICJ will decide that genocide does not mean that anyone can intervene at all without UNSC approval.

This also what made the NATO bombing of kosovo illegal. NATO did not have a right to intervene, but at the same time not many countries felt like condemning them for breaking international law in that case since they indeed stopped a genocide.