How do we handle this?

We are in the process of finalizing the adoption of our 10 yr old foster daughter and her 18 mos old 1/2 sister. We have had custody of both for over 15 mos. Everything has gone just wonderfully, and we have maintained sporatic contact w/ bmom (after her girls were removed, she voluntarily gave them up. Her rights were terminated this past fall). We hear from her every couple of months and see her about every three or four. Bmom is in and out of recovery, bi polar and homeless. My 10 year old lived w/ her off and on her whole life until she joined our family when mom realized she couldn't provide a good life for her and could only take care of herself. Our visits have always been positive and pleasant, but tonight something new came up. After talking to her mom, our daughter wanted to know why we couldn't give her mom $ the next time we see her (which is tomorrow) since she is homeless and poor. In fact, we spent the day buying new furniture for our daughter's room. She said she didn't want it, that she wanted me to return it and give her mom the money. I told her this wouldn't happen, and that her mom wouldn't want her to give her $, that what truly makes her happy is that she is now going to have a great life full of promise w/a family that loves her etc. I am in contact w/ bmoms family and they have stressed from day one not to give her any $, that she enevitably uses it for alcohol or gives it away to other homeless people. We have no intension of ever giving her any financial support. Our relationship will consist of letting her see her kids every few months, but how can I express this to my daughter? Now she is calling me mean and uncaring and "not fair" and I don't know what to say? Mom has never come out and asked for $, but she has jokingly made mention of us adopting her too (ha ha) so we can be "one big happy family". Plus, in the past she has always made light of her situation, now she is sharing w/ me more how destitute she is. She is 40 yrs old and has lived this way for most of her adult life. We really can't and won't help her. Any advise on how I can present this to my 10 yr old would be very helpful. Tonight she begged us w/ tears to help her mom get a place to live and all we could say was that would never happen. Please help. Thanks