Hi. Both my husband and my nephew are in similar situations.
My husband has 2 half brothers from his Dad's first marriage, he's from the 2nd, and he has a step-brother from the 3rd.
My nephew has 4 half sisters from his mom's first marriage, he is the only child of my brother-in-law, and he has a half sister from his mom's current marriage.
Its all been a bit complicated to explain to my kids, but we try to talk openly about who's who as far as a family tree goes, how families are made up of many different people all with different life stories, but on a day to day basis, we don't use the labels. Everyone is just a brother, sister, or uncle. No halfs, steps. Everyone treats everyone like a 'whole' blood sibling.
Sorry if this doesn't help.
M.