Is it a sin to tell a lie?

Sometimes yes, sometimes no.

Everybody knows this as everybody tells lies sometimes and continues to tell lies. There are certain kinds of moralists who get incensed at the suggestion that it is ever okay to tell lies but even they are usually hypocrites about it. In this case, hypocrites where it would be better to do as they do and not as they preach.

The most infamous hardliner on lying was Immanuel Kant who insisted that if Bill asked if he could hide in your basement and then an obviously crazed person showed up a little later holding a big hunting knife and asked you if Bill was hiding in your basement, you shouldn't lie. We laugh at Kant about this but he is just being consistent. If you think morality should be defined by rules, that's where you end up.

A lot of Christians end up in the same stupid place Kant did even though there is no commandment forbidding lies. The commandment is quite specific: it forbids bearing false witness. You can interpret false witness in various ways. It need not mean just in court. It could include giving a recommendation, for example. But no matter how we define it, bearing false witness does not mean lying.

I understand why some might balk at this. No matter how carefully you define "bearing false witness" it isn't going to be enough to exclude all the mean and nasty ways you can hurt someone by telling untruths.

To which, I will simply say, that's fine with me. I think it just proves how limited an idea rules are. Now you go ahead and define "lying" so specifically that it doesn't include any of the necessary lies you need to tell to live.

Rules (and consequences) will always have a place in morality but they can't be the basis of our morality if we want it to be workable.