No. We do not have the time or space here, Peepers, to go into an extended explanation of Catholic philosophy, but it is not entirely possible to understand Catholic moral theology without understanding the underpinnings of philosophy as well. The thread has already been derailed off of several different sections of track.
The matter of sin is consistent, and stable. There is zero relativism about the definitions and categories of sin. But just as a child cannot be responsible for burning a house down if matches are left lying around and there is no supervision, so an adult who is unaware of the danger of telling a particular lie to a malicious, murderous political party is also not responsible for consequences which he cannot foresee. In the case of the child, the person who committed the grievous, mortal, confessible sin is the parent/guardian who left the child unsupervised and within temptation.
The missing link that I did not provide for you earlier is that adult Catholics (for example) are required to inform their consciences. To say, ‘Gee I didn’t know that leaving my child unattended was a serious breach of the Fourth Commandment [a reciprocal commandment, by the way, relating to mutual duties of roles in a family], therefore I needn’t confess it, as no sin was committed,’ does not get off that easily. Here’s the grievous sin such a Catholic committed: not informing his conscience. Unknowing Catholics are committing mortal and spiritually fatal sins to blow them off by claiming ignorance. (There are lots of examples of this on CAF and IRL.

) There’s innocent ignorance, such as a newly catechized member of the Church, or a child incompletely catechized (or below the age of reason); and there’s willful or neglectful ignorance, which is the majority of ignorance. Children are dependent, but adults are not.
As to the third ingredient of mortal sin (full consent), that is also not relativism whatsoever. It accords with Catholic philosophy and the general moral theological base, which is based on the will being crucial to responsibility. The same applies in civil and criminal law. Notice that that is the very principle at play in signing a legal contract: active consent. If that signature is missing, or a person is signing it under duress (threat or other coercion), the will is not cooperating with the activity.
Relativism is not based on the necessary dynamic of moral decision. Relativism posits that there are no stable moral categories. (“Truth is whatever I want it to be; sin is whatever I decide it is.”)