This could be a sign that she's beginning to withdraw from life and (if you're a believer), transition to the next phase. She may also stop eating. A hospice consult would be in order to make her last months/weeks as comfortable and happy as possible.
Why don't you call hospice and have them come out to see if she qualifies for their services? They will be better able to answer any questions you may have regarding your MIL.
Sleeping a lot towards the end can happen. A loss of appetite too.
Is this person in your home or living in a facility? Call a doctor or a nurse if you have questions about their condition.