she is my best friends(friend) and my good sister.

[keannu]

She is my role model because she is positive and patience. So I want to be like her. Now she is my best friends(friend) and my good sister.
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Even if you really like your cousin and have a great gratitude for her, can you say "she is my good sister" when she isn't actually your sister. Wouldn't it be better to say "she is like my good sister"?

 

[GoesStation]

Some people refer to others who are not related to them as sister, but it's not normally possible. Say she's like a sister to me. I wouldn't add the adjective "good"; just sister​ is enough.

 

[Charlie Bernstein]

She is my role model because she is positive and patient. So I want to be like her. Now she is my best friends and my good sister.
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Even if you really like your cousin and are grateful for her, can you say "she is my good sister" when she isn't actually your sister? Wouldn't it be better to say "She is like my good sister"?

GS is right.

We would say "She is like a sister" or "She's like a sister to me."

In very informal U.S. conversation, we sometimes call friends brother (or bro) or sister.

 

[keannu]

Why is this improper? a should be removed as gratitude is uncountable.

have great gratitude for her

 

[GoesStation]

Why is this improper? a should be removed as gratitude is uncountable.

have great gratitude for her

Please indicate the phrase you're asking about by surrounding it in quotation marks.

It's just not idiomatic to say you have gratitude for someone. You can be grateful to them, or appreciate them, or appreciate what they've done.

 

[SoothingDave]

When you say someone is your "good sister," it makes me think you have a "bad sister."

 

[Rover_KE]

Maybe he does; a lot of sisters (and brothers) treat their siblings abominably.