I've dealt with this kind of thing for years with my AW, even before she started drinking heavily. We only have one child, so playing favorites isn't a problem. Spoiling her always has been, though.
Once I came home and discovered that AW had gone out and bought one of those above ground pools, without even consulting me. By the time we had it installed, it cost us $5,000. Years later we had to pay some guy $200 to haul it away.
I guess some people are just that way... maybe those who are prone to addictive behavior are also more indulgent toward their children. Could it be a general lack of responsibility?