I doubt it even has any thing to do with you, or your step bro...think the main reason for his insecurity is because now he is no longer in the "boss" role at work and so maybe is not feeling his manhood any more?
You shouldnt have to proove yourself to him, if you have never been unfaithful, nor given him a reason to feel you have-then he should accept that or leave it, no need to let him invade ur privacy constantly by sharing the tx messages in your phone. I wouldnt condone that, if it was me....& he happened to check my phone when I wasnt aware-then thats his bad-but I would never say Okay huni, u think Im being a slapper, check my phone out daily and keep tabs on me like Im your possession....wat ever!
why did he swap jobs?
why did you guys split in the 1st place?
If ur back with him, you obviously wanna be with him regardless...so he just needs to suck it up-an insecure man is soooo not sexy...