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..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.
