You can certainly make one for kicks. It might work for a small cubby under a desk, but like your sis, I would worry about the open, basically unattended flame. They are kind of cute, but unless you have the flower pots and the nuts, allthread and washers lying around they won't be very cheap. I don't know how much you burn candles, but if the wax does not have somewhere to go, the flame will drown in the melted wax. I just don't see this type of candle being able to burn more than about 3 hours or so or put out enough heat to raise the temps any significant amount. Don't know if I have seen a 50 hour candle small enough to fit into that flower pot. I also would be quite worried about having an open flame that was unattended
Are you running your lights 24/7? The open flame would preclude this from being used during flowering, so am assuming you are speaking of vegging. I always like to add more light rather than a heater if it is too cold during lights on.
It is getting cold here too--will be setting up my "winter set-up" over Thanksgiving weekend. I use the heat from a vegging tent to heat the flowering space during the winter. I have a 32 x 32 tent (on loan to a friend right now) that I set up next to the flowering closet. A large bathroom type exhaust fan moves the hot air from the veg tent to the flowering tent.