It’s wrong, mostly.
The sacraments are invalid: the eucharist isn’t the eucharist, the priests have no authority, the penance is not efficacious, mainly because the orders are invalid (and the sect is schismatic and heretic) and thus the priests are not actually priests. The Episcopal sect (it’s not a church: only RCs and EOs have that respect) is what I believe I know as Anglicanism, and it’s been oft-discussed how wrong the Anglicanism is in many things.
However, to merely sit in the church-building during a service is not a sin, I don’t believe, as long as you do not believe it to be valid, do not hold it in the same regard as a real Mass or Divine Liturgy, and do not go to the Episcopalian service in detriment to your attendance at a proper Roman Catholic Church: however, to go to the Episcopalian service when a Roman Catholic Church is available on a Sunday I would believe is a sin, as it is obligatory on all Catholics to go to Mass on Sunday.
It seems that just sitting through the service, on a day other than Sunday, or after you’ve already gone to Mass, is no more of a sin than listening to a Methodist minister preach, or reading a defense of Calvinism (I’m not sure if more conservative position would hold this to be a sin, as the books would never be granted nihil obstat and imprimatur, as for obvious reasons, read without discretion, they can lead one astray and cause grave damage to the faculty of sacred reason and the faculty of conscience or morality): if you know they are incorrect, yet have interest for educational or research purposes, they can have some value (with the aforementioned caveat).
However, I am not certain about the above, but I am certain that no services are an adequate replacement or proxy for the Mass or Divine Liturgy: there can be no other. To try to defend that all Christian services are the same - or that any service is as valid as any other service - is slipping in to latitudinarian indifferentism.