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.

[![enter image description here][1]][1]

**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.

[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/fktcz.jpg
  [2]: https://i.sstatic.net/DU9IC.jpg