Is it permissible to marry before the elder sister, and is there any offense to her in that?

You are permitted to marry before your elder sister, and there is no legal impediment to this, nor should it affect your sister's feelings. Each person has their own destiny regarding marriage, and delaying marriage can lead to problems. A weak hadith transmitted from the Prophet, peace and blessings be upon him, states: "O Ali, there are three things you should not delay: prayer when its time comes, a funeral when it is present, and a single woman when a suitable match is found for her."