Re: Actions speak louder than words right?
Of course your age matters! It makes all the difference in what sort of suggestions any of us may offer, altough I assure you, few will agree with your method of taking revenge on her by rubbing gas on her dog. Of course you have a right to feel frustrated over your brother's irresponsible actions, and it's not hard to believe that talking to your mother will not resolve anything, you wouldn't be posting here if the answer were that obvious. But what you are doing is cruel, period.
The simple answer: If you are old enough to hold a job, then move out and get your own place.
Simple challenge: Remove the container of gas yourself and put it where it belongs. You are not doing this to alleviate your brother from his responsabilities, you are protecting your animal from getting hurt, in the same way that you would protect him if you took him out for a walk and someone had left poisoned dog food out. You would pick it up and steer the dog away from it.
I suspect there is a lot of unresolved anger in that household.