From Surah Taha
Rabbi Ishrah Li Sadry, Wayassir Li Amry, Wahlul ‘Uqdatan Min Lisany, yafqahu Qawly. (My Lord, I ask you to expand my breast, make my task easy, undo the knot in my tongue so that my speech will become comprehensible) (Moses’ prayer)
asalam alaikum wr wb,
First of all, as well as her consent it is the right of the bride-to-be ask her potential husband to settle the Mahr with her. It's her prerogative to ask for as much Mahr as she wants, but it is recommended that she should not ask for more than her potential husband can easily pay. Has this been done; since consent and the settlement of Mahr are the basic requirements of Nikah?
Secondly, strictly speaking an islamic wedding should not be such that it needs months of planning and finance. I may be wrong but planning of this type usually points to an extravagant affair. The Prophet instructed muslims to keep weddings simple and dignified. That doesn't mean there can be no joyful activities such as singing and dancing, but intermingling of sexes is forbidden.
Given the above conditions of marriage, the sister has a right to be involved in the wedding arrangements. She doesn't need any other excuse, since the above reasons are valid enough. She has the right to refuse/object to plans that include anything impermissable. As a muslim it is her duty to refrain and help others refrain from anything that goes against Islam. Therefore, she can demand to know what is going on. That will help her decide whether she needs more time or not. However if she is satisfied there is no reason to delay the marriage.
wa alaikum asalam wr wb.