Hi Marie
I had a nasty rash when I started to take lenalidomide as part of my triplet induction combination. That was very itchy and raised and quite alarming. I had to leave a longer gap between cycles but was OK to continue the drug at the same dosage and didnt get any further rashes. You havent specified which drugs your step father is on, but I’m presuming that one is an IMID, either Thalidomide or Lenalidomide (Revlimid)?
I suggest that he or your mother contacts his myeloma nurse to discuss this rash, its important for the team to know whats going on, especially when they are not seeing patients as much due to covid. Hopefully the rash will calm down with time and the creams suggested, but as it is getting worse, do ring again.