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. However, 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 does definitely not look good at all.