I'm not saying you need to have her in your party but I think you need to do some serious self reflecting and consider reaching out to your sister again in the future.

Thank you!! This is what I was thinking after reading this. OP's sister sounds like she has seriously debilitating issues. She might be entitled and naive, but if she hasn't personally done anything egregious beyond being born as the second child who was treated better than you, I suggest re-evaluating your relationship or thoughts about her. That doesn't mean you need to be BFFs or super close, but you should maybe consider for a moment the circumstances from her point of view.

Your mother, however, is entirely another story. She sounds entirely unreasonable and potentially toxic. Talk to your Step Dad. Don't compromise. I would guess that those who would want to get married in nature would be unhappy with a church wedding.

Be smart, use your money elsewhere. Also, take a moment to realize how unhealthy your sister is and how she's been constantly put down by your mother -- or enabled and manipulated to an extreme. Seriously, your sister needs help -- even if it's not from you.