I vote with your sis. I think he responded to your text because it was friend-focused. You were checking on him and he appreciated that. Perhaps later he can consider you something other than a friend. The check-in was kind (despite ulterior motive - ha!) but I think now you should let it lie.
If he thinks about texting you in the future he will look at his phone. And see the last conversation you had was one where you checked on him, but respected his wishes to back off. The ball is firmly in his court, with the door wide open.
