You can't. There is no "default". If it's not clearly stated, you have to ask. Generally, if it's not clarified in the text, it's probably not important.

This may seem odd from the point of view of someone coming from a language where the difference is part of the terminology used but as with many familial terms like grandmother/father, cousin, or nephew, only the direct relationship in English is there without adding modifiers.

• My mother's mother -> my grandmother on my mother's side
• My father's brother -> my uncle on my father's side
• My father's brother's son -> my cousin who is the son of my father's brother

It gets a bit wordy but, there you have it. We don't really have a better way of doing it.

When it comes to aunts and uncles by marriage - the spouse of your parent's brother or sister - one might use "uncle-in-law" but (as a native American English speaker) this seems silly and would likely be something I would only do when joking or teasing that person... and it still doesn't solve the problem of whether it's your father's or mother's sibling's spouse.

You can't. There is no "default". If it's not clearly stated, you have to ask. Generally, if it's not clarified in the text, it's probably not important.

This may seem odd from the point of view of someone coming from a language where the difference is part of the terminology used but as with many familial terms like grandmother/father, cousin, or nephew, only the direct relationship in English is there without adding modifiers.

• My mother's mother -> my grandmother on my mother's side or maternal grandmother
• My father's brother -> my uncle on my father's side or paternal uncle
• My father's brother's son -> my cousin who is the son of my father's brother

It gets a bit wordy but, there you have it. We don't really have a better way of doing it.

When it comes to aunts and uncles by marriage - the spouse of your parent's brother or sister - one might use "uncle-in-law" but (as a native American English speaker) this seems silly and would likely be something I would only do when joking or teasing that person... and it still doesn't solve the problem of whether it's your father's or mother's sibling's spouse.

You can't. There is no "default". If it's not clearly stated, you have to ask. Generally, if it's not clarified in the text, it's probably not important.

This may seem odd from the point of view of someone coming from a language where the difference is part of the terminology used but as with many familial terms like grandmother/father, cousin, or nephew, only the direct relationship in English is there without adding modifiers.

• My mother's mother -> my grandmother on my mother's side
• My father's brother -> my uncle on my father's side
• My father's brother's son -> my cousin who is the son of my father's brother

It gets a bit wordy but, there you have it. We don't really have a better way of doing it.

You can't. There is no "default". If it's not clearly stated, you have to ask. Generally, if it's not clarified in the text, it's probably not important.

This may seem odd from the point of view of someone coming from a language where the difference is part of the terminology used but as with many familial terms like grandmother/father, cousin, or nephew, only the direct relationship in English is there without adding modifiers.

• My mother's mother -> my grandmother on my mother's side
• My father's brother -> my uncle on my father's side
• My father's brother's son -> my cousin who is the son of my father's brother

It gets a bit wordy but, there you have it. We don't really have a better way of doing it.

When it comes to aunts and uncles by marriage - the spouse of your parent's brother or sister - one might use "uncle-in-law" but (as a native American English speaker) this seems silly and would likely be something I would only do when joking or teasing that person... and it still doesn't solve the problem of whether it's your father's or mother's sibling's spouse.