This can be many different things from scabies to allergy or even a fungal infection.
You will have to take your cat to an other vet for a second opinion,The vet you are using now could have done a better job diagnosing and treating your cat.
It looks to me like this might be a fungal infection,if it is then your other cat is at risk of getting it but i am in no way saying it will get it only that there is a risk.
It does not look like it is an allergic reaction to me but only a vet can tell this for sure,your vet might have given the right treatment my guess is that the cream might have been an anti fungal one.
Your vet will probably have to run some additional tests to find the cause for this,that ear do definitely not look good at all.