How we act is about us, not other people and what they are doing. That you feel you yourself respond in an "emotional", "insulting" "flippant" and "caustic" way says to me, that you don't like yourself much? You can see when other people act in ways you do not like but not that you do not like yourself?

Pretend you are the other person and the man you love has convinced you that yoga is of the Devil. Do you have a problem if that is what you are convinced of? No, you do not so how can it elicit negatives from someone else, especially your sister who loves you?

Who made you interpreter of what others should/should not believe?

Now, pretend that you are condescending and insulting, believing others are stupid (including your sister, as, if she were not "stupid" she wouldn't have chosen a stupid boyfriend?) and it is your job to tell them what they should/should not believe? Feel good? Again, no.

What we do not like is what we do not like in ourselves. Our opinions are from our thoughts and feelings and they come from within us, are not imposed from outside. You are making interpretations of what you see, think, feel, etc. and when we decide something is "stupid" it is about us and our perception, not about whether something is/is not stupid (no matter how many people agree may with us).

When I get too over-the-top, I stop. It is a warning to me when I get that worked up that something is not right within me, that I feel threatened in some way. Think about it without the emotion, at all. What difference does it make if your sister does/does not have Halloween decorations? None, really, but if we grew up with decorations and someone comes and convinces those around us to not have them, that is threatening to our personal world. We "expect" to live in a world where everyone loves Halloween because that's how we were raised/how we have lived our life. We think we see/understand the world around us but we do not quite understand ourselves and where we are coming from. Extreme religious beliefs to us don't make sense so we assume they do not make sense, literally. But "different" is not "bad" or threatening. It is only our perceived control (we do not actually have any) over our surroundings that are threatened and, sometimes, that is good.

I dare you to go and talk to this guy have a "real" conversation to get to know what he believes and why, with an open mind. Learn about his background, what he does for a living and what he hopes to do in/with his life, etc. I bet he isn't all anti-Halloween/yoga? He might have a wicked :-) sense of humor or love children, work hard at charitable works, etc. But focusing on just what we do not like, instead of looking for what we can like about another, only trains our brains to think of ourselves, what we know already, and negatives? See if you can learn more about this guy, from him, and find something to like? It could be that the Halloween decorations and yoga fit into a more cohesive world view that you don't object to completely and the minor things like decorations and yoga don't seem so angry-making to you?