What is the ruling on a wife saying "my darling, my life, my eyes, and all my world" to her husband in front of her mother and brothers? And is this considered a lack of modesty?

The default ruling is that a wife is permitted to address her husband using certain expressions in front of her family, as long as no religious prohibition results from it, while observing customs and traditions. So, if the expression violates modesty according to the family's custom, it should be avoided. As for the brothers and sisters, these matters should not be a cause for dispute, and they should abandon argumentation and choose good speech, in fulfillment of the Almighty's saying: "And tell My servants to say that which is best. Indeed, Satan incites [discord] among them. Indeed Satan is ever, to man, a clear enemy."