To use comma or not

With the comma, the clause before the comma is a complete statement. The clause after the comma then provides more information about it. It's equivalent to:

I have a sister. My sister lives in England.

Without the comma, the second clause is a defining relative clause that modifies the immediately preceding noun phrase. It could be part of a larger sentence, e.g.

I have a sister who lives in England, and another sister who lives in America.

Perhaps it's easier to see the difference in a sentence where the relative clause is inserted into the main clause:

a) All the residents, who had been woken by the alarm, managed to survive the fire. b) All the residents who had been woken by the alarm managed to survive the fire.

Sentence (a) uses commas. It helps if you imagine the commas were parentheses. The information in the (relative) clause between the commas is, therefore, just 'by the way'. Outside the parentheses, the independent clause reads: "All the residents managed to survive the fire", and this is exactly what this sentence means i.e. there weren't any residents who didn't manage to survive.

Sentence (b) has no commas. So here the information in the "who" clause is not "by the way". It is an essential part of the sentence and, in this case, it defines which residents we mean: only the residents who had been woken by the alarm survived. There were other residents who hadn't heard the alarm and weren't so lucky.

To go back to the original examples: the comma in the first sentence shows that the information in the relative clause is "by the way": "I have a sister, and, by the way, she happens to live in England."

In the second sentence there is no comma so the info. in the "who" clause is not "by the way", it is essential - it defines the sister: "I have several sisters: let me tell you something about the sister who lives in England."