A fetish is about finding an object sexually arousing. It could be a woman's panties, bra, lingerie or any part of the human body.
Your bro-in-law, aged 17, does have a severe foot fetish, more like an obsession. The good thing is that he is not alone and there are many men who worship their partner's feet. The problem here is that he is not your partner.
The love for a woman's feet is also coupled with the need for dominance. It is a great turn on to kiss the feet, to have your feet trampling him and in a way to worship your feet. You must be enjoying this dominance over him. You would rather prefer your husband doing this, but seems he does not.
In a comment you have said, that your husband has seen him pressing your feet. What explanation did you give? How does the foot massage take place? Does he sit on the floor at your feet? How are you dressed up when he does this? In pants or in a dress?
Make no mistake, fetish is sexual, but since he is relatively young, he cannot give full vent to this. You are partially to blame, for leading him down the rosy path. But if you are in control of yourself, then I can only say it is a harmless past-time for you.
He will kiss your feet at some time, if you allow him to continue. I am sure he masturbates thinking of your feet and imagines your feet massaging his cock. If you feel this is fine, go head. Let us know when it gets out of hand.