When A is called D, he is usually called junior brother. After all, A’s master is older, and his apprentices are also older. However, they are also ranked according to the age of entry. This situation is more common. Whoever has been in the apprenticeship for a long time is the senior. For example, a 10-year-old child A was accepted by Master B at the age of three. Another twenty-year-old D was adopted by master C when he was fifteen, so this twenty-year-old D should call the ten-year-old A his senior brother. There are also rare cases of who is older and who is the senior brother. This situation occurs in small sects. The same goes for the second question. The above criteria are also true for women. The older is the senior sister and the younger is the younger sister.
Then there are also situations such as direct inheritance and non-direct inheritance, clan and branch families, blood relationships, etc.