HI Kaj123 what a situation you are in. I am sympathetic and it sounds a tricky situ.
I don't know what advice I can give you. Maybe counselling as a family? From what I am reading, it sounds as though no problems can be discussed in order not to upset your dsd, so everyone has to live in an atmosphere. 
Although I do feel your dsd is in a bad place right now, and quite rightly needs love and support, as you say, it cannot be to the detriment of everyone else. Afterall, it must be having a great impact on the son as well. 
This is why I have suggested counselling as a family. This way you are all on neutral ground and issues tend to flow out better even though it may cause a few tears at first. ( We did not do this at it may have helped in hindsight)
You right that your dp has not helped by letting dsd do as she pleases. This underminds everyone in the household.
You have come here for advice so please ignore negative comments as they aren't going to help you.
I can only say all this as I am fully aware how much one persons actions effect the whole family and it is just accepted.
It has resulted in my dss, even though he is 19, moving out. And I will get flamed for this, but things have made a complete u-turn on everyone, dp included.
He will be the first to admit that he overcompensated for the fact that his ex wife left, letting his son do whatever he pleased.
Then when he tried to enforce some rules and boundaries (after I could not cope with dss attitude as it effected my children and our relationship was falling apart) Dss left for hurt factor and because he was defiant towards me and eventually dp. 
IMO you need to sit dp down and explain this to him. Try not to instigate dsd in the conversation completely as it will just make him want to defend dsd. Just explain how everyone is affected, and how you want everyone, especially dsd to be a happy family unit and how this this and this may actually help her depression.
Good luck and let me know what happens 