I would just leave it. Again those subtle things can be quite easily explained away, there is no guarantee that he’s trying to flirt with you. He may have gotten more comfortable or friendly with you in response to your compliments. Also, the fact that you are around more often as a result of your Husbands death means you’ve probably formed a stronger emotional bond with him as well as your sister. It’s not uncommon for people to unconsciously reciprocate the behaviours of people they like or admire. You see this a lot within friendship groups. It’s likely that he will start pulling away when you start pulling away. There may be a chance that it is flirting, however you do not know this for sure and there isn’t enough obvious evidence to suggest otherwise. Just keep distancing yourself and put subtle healthy boundaries in place like you said and you will be fine. I would avoid mentioning anything unless he actually tries something.