Criminal Sentence 2: It's vs. Its

"Due to the proximity of it's flamboyant sister city, London, Edinburgh often gets overshadowed."

"It's" is a contraction of "it is," whereas "its" is an adjective known as a possessive adjective. Possessive adjectives such as "hers" and "his" never use an apostrophe. It's easy to get them confused if you're not paying close attention.

Here's how to test if you're using "it's" and "its" correctly:
If you can substitute "it is" for your "it's" or "its," then "it's" is right! If not, then use"its."

"it's flamboyant sister": You can't write "it is flamboyant sister," so it should be "its."

Until you're sure you're using these two words correctly, I recommend doing the little test to check yourself whenever you see an "it's" or "its."