Is it considered deception or inappropriate behavior not to inform one's parents about an engagement, and to postpone telling them until their financial situation improves, and out of consideration for the feelings of the elder unmarried sister?

It is narrated in the Hadith: "Three things are not to be delayed: prayer when its time comes, a funeral when it is ready, and a single woman when she finds a suitable match." It is preferable for a girl to accept a young man of religion and good character without delay, especially if she is older, and the presence of an older sister should not prevent it. The older sister should be content with Allah's decree and love good for her sister. There is no harm for the younger sister to consider her older sister's feelings and wait for her marriage, provided that no relationship is established with the suitor before marriage.