I suggest that you need to back off. As a bridesmaid you should not be the one making decisions about the wedding. This is the responsibility of the bride and her bridesmaid. If the bride doesn't want you to wear navy then you don't wear navy.
It sounds to me that you're taking over when you should just be along for the ride. You do have responsibility for the shower but that's all. And.....the shower is for the bride. You do what the bride wants to do.
Also know that since the bride and bridesmaid are twins they have been close and making decisions together all their life. You are an outsider and I suggest that much of your difficulty is related to that feeling.
Back off. When they ask for your opinion give it but don't expect that they will always agree. This is the brides's party. She has the final say. You are not part of the planning, except for the shower.
As for paying for the dress. That is what many bridesmaids do. You should not be paying for anything else related to the wedding. If the bride and her family cannot afford what they're planning then they need to change their plans.
The same for the shower. You contribute what amount you can afford, knowing that the bridesmaid is going to do what the bride wants done. If you paying $1800 means for you that you want more control, then withdraw the money. Realistically you cannot expect to have more control. This is about the bride and her family. It's good that you're willing to help pay the cost. It's not working for you to expect so much control.
Ideally, you would have more say. But, that is not happening here.
I suggest that $3-4000 is outrageously too much for a shower. Or are you also contributing to the wedding? If so, since you expect to be more involved in the planning take back your money. Or adjust your expectations to the reality of the situation.
A wedding is all about what the bride wants. She is in control. Either accept that, being willing to go along when the bride and her mother feel it's important to do certain things or say, up front, that unless you have more control you feel that you're unable to spend that amount of money.
There is nothing wrong with the way you feel. I, too, would want to have more say in the shower. But that isn't happening. You cannot change them. You do not want to have hard feelings in the family over this. I would, as gracefully as possible, withdraw from the planning and probably reduce the amount I was willing to contribute.
I'm having a similar situation with a friend, not about a wedding but about helping her with health issues. She is having senility issues and wants to tell me how to help her even when the help would be detrimental. After months of anquish, I told her what I could do and have stopped trying to help her understand what she needs. What a relief.
It has taken a great deal of diplomacy to do this and still maintain our friendship. What has helped me is to have a professional person help me plan the wording and give me emotional support. I had to stop expecting my friend to be reasonable and logical. I have to focus on me and my needs first and then think about her needs. I have had to accept that she doesn't think the way I do and will not understand my viewpoint. Then I had to stop trying to convince her I was right.
I said, "you're right. You are in control of your health and how you accept treatment. I don't agree that you're doing what is best for you. I see you forgetting what the doctor tells you and want to help. You're not accepting my help and so this is what I'm able to do and still maintain my sense of well being."
Perhaps you can use that as a model for how to talk with the twin sister.
Remember the only place that you share responsibility is with the shower. The bride decides what dresses the bridesmaids wear and everything else about the wedding.
I also suggest that you find a way to be assertive. The way you've written this makes it sound that you've been handling this in a passive agressive manner. When you don't agree and are not willing to compromise be upfront and direct and say, no. You might be able to salvage this by first making a list of what you are willing to do and not do and hand the list to the sister. Know in your mind what you're willing to pay for. Be willing to discuss things but say no when you know you're not willing to compromise.
I suggest that the sister is saying out front what she wants and your'e feeling pushed and manipulated. Stand firm in a kind way without having to get angry. Know what you want and stop thinking that you have to cave or be angry.