The term 'disloyal' competition is a slight mis-translation into English; consider it to mean 'unfair' competition (UC).
There are many different strands to what constitutes UC, and the different EU states have a varying degree of protection against it (Germany are especially protectionist).
To be quite curt, you're in the legal wrong here. What you're doing is quite possibly 'reverse passing off' (you hold out Brand Y as yours or in some way affiliated with Brand X when its' not); even if not, it is still probably UC. By attempting to use another's goods and reputation to enhance your own sales is a clear case of UC. (In the USA, even using mirrors in your store to hide your neighbour's shop can be UC!)
Best thing to do: send a polite email apologising and feigning ignorance of the law, then remove the content ASAP.