I would continue buring the lights in good faith and try to put any possible negative effects of her extinguishing them improperly out of your mind. I'm sure many here have blown out an unfinished candle by mistake during spellwork and gone on successfully with the working. I cannot believe the spirits would hold her actions against you, a true and faithful believer.
Your kindness is apparent: from reading your previous postings and the fact you have taken in this relative in her time of need. Your adversary's lack of kindness/understanding is just as obvious -- I think you are right, there is probably no sense wasting your energy attemtping to change her behavior. I do not mean you shouldn't do work to protect yourself from her, but that there's clearly no honey jar sweet enough for the ol' vinegar pot!
As distasteful as it may be, I think those who are suggesting stealth are probably right on track. If you continue to work with the vigil lights, by all means, be careful; if her mind is as warped as you believe it is, she may arrange a candle fire just to vindicate her sick mind. (Look! See the outcome of those evil and hazardous candles!) NOT what you need! From personal experience, I can say I really like the 4" candles; they burn for 1.5 - 2 hours. Which for me means I can do focused candle work every morning/evening if I like and not have to worry about my bad kitty or curious children burning down the place by accident. If you feel very strongly about having a vigil light burning for a certain situation, you may want to look into having Ms. Cat set a light for you on her altar; I've heard very good things from others who have used this service.
Take care, friend, and let us know how you fare.
Michelle Hunter, a.k.a that Tricky CopperFox
I will lift mine eyes up to the mountains, from whence comes my strength...