Regarding your examples, "I bought a cup which is similar to the one my brother owns" would imply that you set out to buy a similar cup and you probably would not have bought it if it was not similar. In "I bought a cup, which is similar to the one my brother owns", the similarity is merely incidental and probably played no part in your purchase decision. Regarding construction ("syntax"), a native speaker would probably edit the defining version to read, "I bought a cup similar my brother's." If you had to use "which", then the sentence would be more correct if it read, "I bought a cup which was similar to my brother's." Note the past tense "was", which is necessary if you are using a defining clause. The fact that it was similar at the time of purchase played a vital part in your purchase decision. English grammar is fascinating. The more you learn, the more there is to learn and enjoy.

Yes, it makes a difference whether or not you use a comma. A simple explanation is that the comma signifies that you can "cut out" the relative clause, as it is just extra information in the sentence, but the sentence can function without it. No comma tells the reader that the extra information is vital to the sentence. Look up "defining relative clause"(no comma) and "non-defining relative clause"(comma) for a full explanation of the grammar.

"完全取决于中文你要表达的意思" - you assert that "It depends entirely on what you wish to say in Chinese". It is silly to write English, or to learn to write English, by first constructing in Chinese what you want to say. It is even sillier to suggest that English usage or syntax should depend on what you wish to say in Chinese. The best way to learn to write English is to think entirely in English. To learn English is to acquire a completely different way of thinking from the Chinese way. This also answers the frequently asked question of how to learn English well and achieve mastery in a short time.

英语的限定与非限定完全取决于中文你要表达的意思，中文严密，英语就限定，不严密强调，英语就非限定。 个人意见，不一定正确。我是学生哈。^_^

前者，我买了个杯子，和我妈妈的很像。后者，我买了个和我妈妈的杯子，很像的杯子。