This is the tree which the highest 是有语病的
正确的说法应该是This is the tree which is the highest ,而这样说显得冗余,
直接说This is the highest tree会比较好.
My sister works in a bookshop in which we can read many kinds of books
在定语从句中从句的引导词in which=where,因此要么用in which要么用where,而不能用
in where或者单单用which