Blended Family Woes

Trying to keep it short...

We have 3 children- 18, 15 and almost 2. The oldest two live with their mother and have not slept here since Christmas, despite our best efforts. My mum moved in to take care of the little one while we work and we are just finishing up her basement apartment. Currently she's in the bedroom of the oldest who goes to university in the fall. When he has slept here she sleeps on the sofa or goes on a trip so as not to make him feel like he doesn't have a room and hasn't re-decorated that room. She even puts his bedding back on for him so it's "manly". Question is two-fold. First my little one who is here full time has the smallest room because until now she was a baby. Now she's a toddler and her room is not useful for play. Also at times we work from home and don't have an office space at all. Would it be ok to turn the baby's room into the office with a guest bed and move the baby into the big room since she is here full time? Then we just have one unused bedroom for the 15yo and the 18yo could sleep in the guest room if he comes home from school? The older ones have never kept clothes here (dressers totally empty) and have just turned down going on summer vacation with us, despite our active involvement in their lives. My partner is very sad about how they treat him, and he and is onboard with the re-arrangement of the house I but want other blended family perspectives. Second, We don't know how else to make them feel involved - they have for the last 4 years had their own stuff and own rooms and have no interest in coming here or doing anything with us. We have talked to them about it, we go to every activity and sports event, but they want nothing to do with us and I'm tired of having my littlest with no place to play and paying for a house that mostly stays empty.