Short answer: Yes, they should cancel.
Now the longer discussion. This is another rule I wish they would change (edit). The rule says that when dead ball UNR and/or UNS fouls are reported to the R and none of the penalties have been “completed,” the yardage penalties cancel, and the down established before the fouls is unaffected. “Completed” means accepted, declined, or canceled by rule, or when the choice is obvious to the referee.. (Officially, the next time the ball is ready for play has no bearing on this process.) And therein lies the rub. It should be obvious to the R that the sideline interference is accepted the instant it is reported to him. So, if the other foul(s) occurs, or is reported, after the sideline interference, then the sideline interference penalty should, officially, be penalized. Penalties for multiple opposing dead-ball UNR/UNS fouls after the sideline interference might cancel, if they are all reported to the R before any are completed. But, a single. UNR/UNS would get penalized. That’s the technical issue.
But, more practically speaking, yeah, all of those DBFs should just cancel, and we just move on to the next down.
I wish the definition of ‘completed’ would mean when the R begins to signal/announce the penalty(ies).
