I think it means that Margaret is 13 yrs old, and while one may love a 13 yr old sister, a child at the age is not to be confided in about the loss of one's deepest love, or to be trusted with one's most private secrets. Especially if she tends, as Margaret does, to spill those secrets. Remember Margaret's delight at informing the Middletons and Mrs. Jennings that Elinor's suitor's name began with an "F"?