Assalamu Alaikum Brother,
InshAllah this response reaches you in strong Iman.
I have read your question and put simply, your sister is not your responsibility. If she is married, her husband must look after her and is responsible for her in all regards.
Once you are married, your wife then becomes your primary responsibility (after your parents if course). Why would you expect your wife to accept this, and further more to contribute financially to the trip for your sister (of course, this is unless she, from the goodness of her heart offers to do so). If your wife is working, like another poster has stated, the money she makes is hers and hers alone. It is not for you or your sister to spend.
Secondly, you and your wife are a partnership. All that goes on between you or that involves you must be discussed between you both. This is the key to building a strong relationship and also respect between you both. It is a normal reaction for your wife to be slightly uneasy or uncomfortable with your married sister to be tagging along on this trip. Make dua that your sister is able to fulfill this duty with her husband inshAllah, or her son in the future if she is unable to do at the moment.
In regards to the issue that your sister is making decisions without your knowlegde, and sharing them with the rest of the family, well that needs to stop and you would need to be the one to bring this up in a polite, yet firm manner. This could potentially cause very severe problems between you and your wife in the future. Brother, try to put a stop to it now inshAllah.
Lastly, if your wife accepts your sister coming along with you both, then the reward is with her InshAllah.