The prior relationship between Sanchez and Davila will not effect the admissibility of the testimony of either. The relationship must be disclosed to the defense. The defense is allowed to cross exam the witness regarding the prior relationship in an attempt to attack the credibility of the witness. This was done in this case. It is then up to the jury as to whether they believe all, some or none of the testimony based on how credible they believe the witness. In this case it appears the jury believed Sanchez and Davila rather Ramos and Compean. In reading the testimony of the agents, I can understand how this occurred. The agents were fairly poor witnesses, especially Ramos. IMHO, this is not an area that is a fertile for the appeal.
Thank you for your comments, erton1. I'm not a legal expert by any means so my opinion isn't worth a hill of beans. However, from what I've read on this case, it is my opinion at this time that their appeals will not be successful.