1. The wife of the younger brother called the older brother is also called the older brother, because he married into the man's side and followed the man's side. A brother calls his brother's wife his sister-in-law! There should be no objection to this. If it is the sister's husband, the sister calls him brother-in-law. If the sister is the sister-in-law of the sister's husband, the sister's husband is usually called by her first name or nickname. Call the sister's husband brother-in-law! It is the same for a sister's husband to call her sister and her brother-in-law!
2, Mom's brother is her own uncle. As you said. Uncle/nephew (daughter)
3, mom's cousins & nieces should also be called uncle, or cousin & cousin. Their children are equals to themselves, the elder ones are called brother (sister) and the younger ones are called brother (sister).
4. Dad's sister, it is right to call her aunt. Call yourself a nephew (daughter). There is no objection to that, is there? Her children and herself should be cousins! Mom's siblings are only cousins!
There are some differences between these titles in China, and I sometimes get confused. It's quite a headache. Hehe~~
But then, if family planning continues, a lot of these titles will disappear.