Ping!
/johnny
Is it an express sin as stated in the bible? Than no, it is not.
If you confess, in good faith, to a person you know is a priest, your confession is valid but may be illicit if the priest is hearing the confession without the faculties granted by the local bishop except in cases of emergency. If the priest is hearing confessions without the appropriate faculties, the moral onus is on the priest, not the penitent. That’s the difference between illicit and invalid. The penitent’s actions are valid. The priest’s actions are illicit. In an emergency - accidents, imminent death etc - confession to a priest is valid and licit. Even a “former” priest may absolve in those situations. Canon law always operates to the benefit of the faithful while governing the actions of the minister.
You will not hear heresy within the confessional from an SSPX priest. You will, frequently, from a mainstream diocesan priest ordained and living under the subversive influences of VII. Even to be told that sins you know are sins “are not sins”. I personally would avoid the question of whether the SSPX absolution is valid by going to an FSSP priest, instead, who at least get the liturgy and sacramental traditions right.
I’d be interested to see what Fr. Z has to say about Rome turning a blind eye towards the validity of the countless confessions (heard by) and marriages (witnessed) by the priests of Bishop de Castro Mayer, after Bishop Castro Mayer was supposedly “excommunicated”, along with Archbishop Lefebvre and his four bishops, before Bishop Rifan finally smoked the peace pipe and inhaled the “spirit of the Council”.
Here’s an article on the canonical status of the SSPX, at least back in late 2011:
http://canonlawmadeeasy.com/2011/10/05/canon-law-and-the-sspx/