Compilers and MSB identification for Data types
The order is defined by the compiler. If you look at the  MPLAB C18 C Compiler User's Guide and search for "endianness" on page 12 you'll find:


  Endianness refers to the ordering of bytes in a multi-byte value. MPLAB C18 stores data in little-endian format. Bytes at lower addresses have lower significance (the value
  is stored “little-end-first”).


So in your example *xptr will reference the LSB and *xptr+1 will reference the MSB.