No because:
In serious situations, you can resort to mental reservation,
Mental reservation is not a sin but it is a lie. It is not a sin because of the circumstances of the lie you are telling. It is a lie because you are not telling the truth.
Look. You know that John is in your house. You have seen him and you know he is there.
Case I. John has just committed a robbery and has murdered several people. You know this to be true. The police come to your door. They ask you: … Is John here. You say no.
John is not here. That is a lie and a sin.
Case 2. John is a 12 year old Jewish boy. He is an innocent boy and a decent person. Nazis come to your door and ask you if John is here. You make a mental reservation and say no
John is not here. Your mental reservation is based on the fact that first you do not think that the Nazis have the right to know this and secondly you know that john will be sent to Auschwitz to be gassed. It is a mental reservation which is justified by the circumstances.
In both cases you say the exact same lie:
John is not here.
Case 1. It is a sin because you know John is guilty of terrible crimes and your duty is to tell the truth so that justice may be served.
Case 2. It is not a sin because you are making a mental reservation. Even though you are lying and not telling the truth, you are permitted to make a mental reservation in such a serious case. Although you lie and say
John is not here you add mentally and only in your mind so that the Nazis do not know, you add mentally that John is not here
as far as you are concerned,. But that is not what you said. That is only a mental thought which justifies the use of a mental reservation. You have lied when you said
John is not here but in virtue of the rule of mental reservation under extreme circumstances, you have not committed a sin.
So to sum up, whether or not a lie is a sin depends on the circumstances. Sometimes the lie is a mental reservation, and as such, the lie is not a sin.