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No one can say they are postively invalid or positively valid.  This is the issue - doubt.
If there is reason to doubt - and there is, since new-rome's new rites are systematically ambiguous - then conditional ordination is necessary.  One is not allowed to attend a doubtful mass or sacrament under pain of grave sin, per Canon Law.  Therefore, one is not allowed to attend a TLM by a doubtful priest, under pain of sin.
This is a very charitable thing for you to do, but it's the sspx's fault that you have to do so.  This information should be publically posted for all to see.  Every catholic should demand to know that their priests are valid - with 100% certainty.  The fact that the sspx is scared to question new-rome's sacraments is a symptom of their political games and their lukewarm defense of Tradition.

Two questions. One comment. 
Question number one. If I did not know this about it being a sin to attend a Mass of a doubtful priest and had been attending (not receiving communion) for the sake of family unity would that be a venial sin or not a sin at all because it is done for the sake of family unity (and so as not to send them over the cliff and completely lose the faith)? Kind of similar to the controversial question posed to +Bp. Williamson about attendance to the NO Mass (but now with respect to the TLM celebrated by a doubtful priest). Question two. Now that I know this, would it be a mortal sin or would the above extenuating family circuмstances mitigate the culpability? (I have had trad priests say different things on this subject--so I am very confused). Since these are ostensibly Catholic sacraments wouldn't taking this position be schismatic? (Sorry probably more than two questions there...)
Comment. As for posting information publicly for all to see. Here it is: the priest is Msgr Byrnes of St. Judes near Philadelphia. I don't believe he was conditionally ordained. I could be mistaken, but I believe that is what he said to me a few years ago. He was a very friendly priest and I do not intend to create controversy, but as you said--the faithful have a right to know this (though I imagine this is common knowledge in the SSPX world). 

You cannot truthfully say they are positively invalid as if you actually know, because you don't actually know, because it is impossible to actually know that all NO sacraments are invalid.

I know of a few ex NO priests who joined the SSPX and were conditionally ordained. So while I often agree with your posts, and I know the SSPX upper management is hanging by a NO thread, I cannot agree with this one.

Case in point. Have you heard of Bishop John Elya of the Melkites?
He told me that he goes to every Novus Ordo consecration of a Roman Catholic bishop to which he is invited, and he has been invited to many. Bishop John is quite sure that he has been validly ordained and consecrated through the Eastern Catholic apostolic line of succession. Thus, to insure that Roman Catholic bishops have valid orders, he personally lays his hands on every bishop during their ordination.

How about the validity of Episcopal / Anglican consecrations / priestly ordinations within that particular sect?  Pretty cut and dry when compared to the “Novus Ordo Sect” ordinations” and “consecrations”.  Absolutely null and void.