If Ukraine joins EU [hypothetically, say within a few weeks], becoming a full member state - just before being fully invaded/occupied by Russian forces, EU members won't recognize the 'Land' of Ukraine to be part of Russia.
But by legal documentation, Ukraine is basically a piece of land - and only its representatives (like PLO for Palestine) have changed. Thus, while The EU may be wanting to interfere - isn't this by European International policy, a bilateral conflict b/w Russian and Ukraine?
In that case, the occupants can declare sovereignty which EU has to respect - or risk violating one of its core principles. According to Article 7, there's also no mechanism to remove a EU member. Thus Russia can also enjoy most EU benefits too - because technically even if the legitimacy of the member is not recognized, Ukraine is still a member nonetheless just with Putin as the President rather than Zelenskyy.
This is pretty confusing and convoluted - can someone shed some light on this?