We all want the rats to be removed from our house, is there anything
we can do about it?
From a purely legal point of view, probably not. While the pet owner is clearly violating her rental contract (assuming it forbids pets), the rental contract is between her and the flat owner. That means only the owner can enforce the terms. If they choose to ignore her contract violation, legally there is no way you can force them to enforce the contract.
The only legal options I can imagine would be if your personal rights are somehow violated by the pets - for example, if they smell excessively, make a lot of noise, spread contagious diseases or similar. However, for well-kept pet rats I don't see how you could make that case.
So, practically speaking, the only option is to ask nicely (either the flatmate directly, or the flat owner). If they do not want to do something, you'll have to live with the rats (or move out).