That it sounds much as the students adult fantasy book Out-of A couple of Thoughts by the Carol Matas and Perry Nodelman
More mature cousin starts dating pal’s sis
The two children with mind powers are Lenora, a headstrong princess who can make anything she imagines real, and Coren, a shy prince from a kingdom where everyone can read each other’s thoughts. The two are betrothed by their parents, the rulers of neighboring kingdoms (both made-up places). They are kidnapped and taken to a strange country where their normal powers are gone. Together, they have to defeat the evil tyrant Hevak and save both of their kingdoms. Originally published in 1995, I read this in the late 1990’s too, when it was published as a Point Fantasy paperback. The cover is dark, with a picture of Lenora right-side up in the lower right corner, and Coren upside-down https://getbride.org/sv/hetaste-kubanska-kvinnor/ in the upper left corner. There are also a few sequels, called More Minds , Out of Their Minds , and A Meeting of Minds . Pamela Service, Being of Two Minds , 1991, copyright. Except for the dates, the details match Being of Two Minds by Pamela Service . The two characters that could get into each other’s mind were an American girl and a crown prince of a small kingdom. The prince is kidnapped when the girl Connie is in his mind. Perhaps this is it. S622: sisters at a school in New York Book about sisters, orphans. Older just got a new job as a teacher. Younger enrolled as a student, becomes friends with another girl, who is rich– her father’s wealth began when he found a golden walnut.