So DH and I have created a new budget. I get CS for my child so we budgeted a certain amount for them for things like school pictures, clothes stuff above what we would normally buy like shampoo and stuff. Anything extra comes out of the budget .. For example a pair of boats because DD likes boots during the winter.. Would come out of her extra monthly allowance.
We created a spending allowance for each one of us for the month as well. These is where we can personally loan money or he can buy for fauxSGD and fauxOSD and I can do the same with my brother or newphew. We built in a little allowance for YSD (DHs bio) even though she is an adult if she needs help we help but no longer outside the budget. What YSD doesn't need or we don't spend on her goes in saving. We also budgeted savings amount.
We both feel this will stop any resentment. If he helps faux OSD with fauxSGD it will no longer mess with our budget.. It will give him less spending money. And he likes to spend so I don't see it happening much anymore