With the latest chapter of 369 we have even a few more clips at the relationship of the Ice Devil-Slayer with gray fullbuster and natsu.
Initial meeting of Silver & natsu
^ Here we see hes obviously not like any of the "demons" in tartaros and seems like a normal human and acts like one. Also natsu recognizes his smell as similar to someone he knows so blood relation is a given.
To confirm it more at grays name he instantly reacts. and he also knows who natsu is speaking of. Basically making the blood relation of Silver and Gray obvious probably his brother.
in latest chap.
shows kindness to fairy tail lisanna could just be because shes cute and naked but hes obviously nothing like tartaros.
Here we see hes frustrated that natsu of all people came there which i believe is inconvenient for him in some way.
The fact alone that hes a devil slayer in the middle of a guild of demons alone pretty much makes it seem like hes infiltrated them and natsu being there could create trouble for him especially if he sees fairy tail as an ally to his cause.
I'm sure many have assumed a similar theory just putting it up for a vote.