I know a thing or two in genetics and (based on their pics) it can go 3 ways:
1. They have only ONE same parent. That will explain similarities AND differences in physical features.
2. One of them is adopted.
3. It's a clear play on fetish, which will attract certain audience. No shame in that on both sides.
Pick your poison.
P.S. There might be other situations, but I'll bet on this 3 🤷♂️. I did have spontaneous experience with 2 brothers (actual brothers and not providers) and that memory will be with me forever 🙃