Wow. Denial!
Wonder what al-anon/co-dependency no more say about the argument that living with an alcoholic isn't damaging to children. Or that drinking every single night and smoking loads of weed and failing to parent is fine. Crazy logic.
There's a thread on here "adult children of alcoholics", for a start. How arrogant to demand "evidence", it's bloody obvious!
Storing the kids' stuff: get a ground floor flat with access to a garden or shed. We used to have a one bed flat in London, with no garden but loft storage and small shed in front garden. No piano, but could've found a space for one. Amazing what you can squeeze in.
Stock for business: seek cheap storage. If it's to become a viable business this would soon be necessary anyway, if it's not viable and just pin money your energy would be better directed to finding more lucrative employment.
Small pets should be Ok in a flat. As for getting a dog, why do that, taking on a big responsibiliity when you have a health issue and have lots on and your P can't be relied upon even to look after his DC?
As for buying your sister's house, your sister doesn't sound all that reliable, and why tie yourself to your DP with a shared mortgage (giant debt)? When he is in a new job that he could be fired from?
Why rely on him financially when he isn't reliable as a father or partner?
Fear of poverty and worse financial situation is understandable, but you may well be financially OK.