Mu SIL's youngest child is leaving for University in September. So it will be just her and BIL for the first time in 25 years. I can understand this is a big life change for her and the empty nest can be hard to come to grips with.
However, SIL is incredibly down about it and acting as if her life is practically over and will no longer have any meaning. I feel very sorry for her but I also think she really never thought ahead to this time while her children were growing and becoming gradually more independent.
She gradually dropped all of her college and work friends as soon as she had children and just replaced them with 'mummy friends' from school and toddler groups, most of whom drifted away as soon as the children got older and no longer wanted to play together. She ignored all suggestions from her husband that she go back to work part time or do some kind of a course to get her out of the house. I asked her a few times if she'd like to come along to an evening class with me or join a gym but she wasn't interested. She basically just invested all of her time and energy in her children, to the exclusion of everything else, and is now totally bereft.
AIBU to think this is a very short sighted thing to do and it's important to remember that your children won't be at home with you forever and you need to retain or rediscover some interests and activities of your own for when that day comes? I hate seeing SIL like this, it's almost as if someone belonging to her has died.