I only have a partial answer.
In Hindu tradition, should you consider your elder brother's wife who's younger to you as your sister or as your mother?
Manu says the wife of one's elder brother is to him like the wife of his Guru neither like his mother nor his sister.
Nothing is mentioned if she is to be treated differently depending on her age.
Manu Smriti 9.57. The wife of an elder brother is for his younger
(brother) the wife of a Guru; but the wife of the younger is declared
(to be) the daughter-in-law of the elder.
Can you prostate to her (i.e., do sashtanga namaskara) as you would to your parents or elder brother? Does your sister-in-law's age play any role in this?
Manu says that the feet of a brother's wife should be clasped daily if she is of the same caste.Here again,there is no mention of age.Also,i think this applies only to the elder and not to the younger brother.Because a younger brother's wife will be like his daughter-in-law.
Manu Smriti 2.132. (The feet of the) wife of one’s brother, if she be of the same caste (varna), must be clasped every day; but (the feet of) wives of
(other) paternal and maternal relatives need only be embraced on one’s
return from a journey.
Update- The Usana Smriti(the Smriti composed by Daitya Guru Shukra) says that the wife of one's elder brother is to be treated like one's mother although the mother is the more venerable among the two.And that her feet should be touched/clasped while saluting.
The wife of an elder brother, those of kinsman and relatives, mother's
sisters and father's sisters, and elder sisters, should be saluted by
touching the feet. (32) One should treat them all like his mother, but
the latter is superior to' them all.
However,this verse should not be seen as contradicting the verse of Manu given above.Because the same Usana Smriti also says:
The mother's sister, maternal uncle's wife, motheiv in-law, father's
sister, and the wives of all the elders, should be adored like unto
the preceptor's wife. (31