On to the rash...
Does the rash make rings or non perfect circles or worm S shaped like lines?
If so... it could be ringworm (which not a parasite) but a fungus type rash which is related to athletics foot but can be on any area of the body. It can flare up and reside on a daily basis. If you picture google ringworm - you will see what severe cases of it looks like, but he might have a mild case of it and it won't be so obvious as the pictures on google show. Ringworm is pretty common.
So sorry for your sister.
About the rash, it’s really hives. Raised up in various shapes usually circular but sometimes its so bad that they run together and form a big huge one. There’s no color to it other than the color of his skin. I don’t think its’ ringworm. I’m familiar with that because my daughter had it when she was a little girl. But i will google it just to be sure.
Thanks.