But is it a worse sin than lying to anyone else, because he is a priest?
Yes, because of the ontological dignity of the sacred Priesthood, but practically speaking in our days...
...not really, because in the eyes of Canon Law, all traditional Priests and Bishops are laymen, for they have no mission, office or jurisdiction, and they are therefore not part of the hierarchy; and this is exactly the juridical position of the layman.
The only difference between the clergy and laity in the traditional realm lies in the power of Orders which Priestly Ordination or Episcopal Consecration bestows upon a man: such power of Orders does not carry with it power of jurisdiction since the Ordinations and Consecrations done by the acephalous and vagrant Bishops are themselves a violation of Canon Law. They do not commit mortal sin because the good of the faithful demand such Ordinations and Consecrations, but validity does not assure jurisdiction: for even the Oriental schismatics have valid Orders, but they cannot form part of the hierarchy of the Church since they do not belong thereto by reason of schism and heresy.
The traditional clergy who are really Catholic (not sacramental terrorists or sectarians) are part of the Church but not part of the hierarchy. They do not have any authority, much less the faculties to impose punitive or coercive judicial decisions.
So, the position of a traditional Priest is that of the layman, Canonically speaking.
Reverence is to paid to the Priest is the Priest shows himself worthy thereof by his paternal solicitude, Apostolic zeal, probity of life, personal sanctity, competence in matters of sacred doctrine and pastoral theology, &c. This is because there is nothing that can assure us that such or such a Priest is competent enough to
de facto demand our reverence and credibility.
Even then, the relationship between the Priest and layman is more like unto a fiduciary contract based on charity and humility, rather than a subordination based on obedience and justice. You are under no obligation to follow them blindly, nor to adhere to them exclusively to the prejudice of other clerics.
You pay heed to the Father Confessor to whom you entrust your soul not because he has authority outside the confessional, but insofar as he is your spiritual director whose counsel has more weight than that of laymen or other Priests.
Go to a Priest you trust regarding your scruples upon this matter, and pay heed to him because you trust him and he knows you and your situation.