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If we agree with the definition from Wikipedia, i.e. that it is "an immoral act considered to be a transgression against divine law," then it would be the divine law (and not us) that decides what sin is and what is not. In such a case the concept of sin would be dependent on a religion (potentially different for different religions).
Does there exist such a unifying concept that works for all religions?
Sin is defined as separating oneself from God. For a polytheistic religion, for example, sin would have to be defined differently.
Sometimes it's clearer if you step back and try to understand exactly what you're asking.
Let's break it down:
Sx := x is a sin.
Vxy := x violates y.
Dx := x is divine law.
Sx <-> (Vxy & Dy)
which means that for all x, x is a sin if and only if there is some y such that y is a divine law and x violates y.
What would then for there to be a unifying concept of sin across religions? It would entail a unifying concept of the relation of actions that violate rules and also at least one instance of a divine law that is agreed upon as divine by believers of each religion. The first criterion is pretty easy because pretty much everyone knows what it means to break a rule.
The second criterion is the real meat of your investigation: Does a rule exist that is considered to be divine divine law that all religions agree upon?
What's a divine law exactly? I'm understanding this to be a rule that some deity (or deities) agree people should obey. The fact that the deity in question is different for different religions isn't really relevant to the question since you're asking about the "concept" of sin. This concept could be parameterized with an abstract deity without diluting the concept, (that parameter would later be filled in with a reference to whatever deity/deities (Mohammad, Jesus, Elvis, etc.) are considered to emit "divine laws" in the corresponding religion). If we are to allow the concept to be further parameterized with a rule, then we have already arrived at an abstract concept, central to all religions that represents "sin". This basically posits your question as the answer, in that there is a unifying concept of sin across all religions, but that concept is defined relative to the gods and rules of that religion.
It may be that you're not actually looking for the unifying concept, but the unifying instance, which would be some action that would be considered a sin for practitioners of all religions. So, in this case we can narrow down the question to an anthropological one rather than a philosophical one. This approach can never practically prove that such a rule does exist (as there will always be the possibility of more religions with other rules), but it could disprove the assertion that such a unifying rule exists by means of a counterexample. That counterexample would be a finite set of rules, where each rule is not considered to be a rule by at least one other religion. I could make up a trivial example, "I believe my cat is God, and the only sin is not feeding her on time" and since not all religions agree with this, we've negated the existence of such a universal rule. That kinda feels like cheating, plus I have a better counter-example that's not contrived. Checkout LaVeyan Satanism - a religion that is defined in direct opposition to mainstream Judeo-Christian thought. You could probably find the schism necessary to prove global inconsistency by contrasting this religion with Christianity.
Now, this wasn't part of your question - but I have an intuition that what you might be after is an underlying moral principle that pervades human consciousness and maybe you are looking at sin and religions in hopes that such an underlying moral principle would manifest in all world religions. If this is the case, then I'd recommend reading Kant's Groundwork of the Metaphysics of Morals, which gives the best purely logical evaluation of morality that I've encountered.
It's not only a question of which brand of religion you ascribe to, but also culture. And it's not a question of being polytheistic or monotheistic, or any brand of theistic. What is considered unacceptable behavior or sinful in one culture may be acceptable in another in the same religion.
For instance, cousin marriage. In some cultures cousin marriage is acceptable. In others, it is not. In some killing of animals is acceptable, in others it is not. In some certain animals can be killed for food, others not. To do so is sin.
In Hinduism, there is not so much the western conception of sin. There are actions which lead one towards God and actions that lead one away from union with God and towards rebirth.
There is no universal definition just as there is no universal religion.
