What type of barn/pasture situation do you have? What was your plan for weaning the filly anyway? How is she with other horses with her mother around? Could you stable/pasture the mare and foal beside the 20 year old mare to see how they all get along together?
How much have you handled the filly so far? How does she respond to being handled? What do you plan on doing with her? Are you going to train her to be ridden/driven?
I would recommend putting the mare and foal in a stall/paddock beside the 20 year old mare or introduce them in a controlled situation. Watch the 20 year old's attitude toward the filly very carefully. Look for any signs that she feels threatened by the filly. Most likely, the mare will either ignore her or just accept her as another member of the 'herd'. However, if the filly is going to be alone with just the 20 year old mare then when/if you decide to do anything with the filly later, you'll likely end up having to 'wean' her again from the 20 year old.
I would suggest that you begin working with the filly to develop her other senses and her trust in humans so she can be turned out with a 'herd' and used as a riding/driving horse. Depending on her personality and mind, she could very easily develop into a good horse or her handicap could limit her to being a companion only. If you work with her constantly and consistently it's much more likely for her to be the former.
Yours is a different situation than I had, but I thought I'd share what I've experienced. We had an Arab/Saddlebred gelding that lost his left eye as a 2 year old. The vet recommended euthanizing him, but we knew he had a good mind, so we didn't. We spent a tremendous amount of time (daily) with him working on his good side and then doing the same on his bad side. Basically we sacked him out almost constantly on that blind side for a good few weeks. We we able to turn him out with the herd of 10 horses (including 4 foals, a couple geldings, & the rest mares) after we had worked with him. He turned into a great horse that could be trail ridden and even jumped by anyone. When he was 22, his good eye got scratched and developed cataracts and scars that prevented him from seeing very well from it. We had a mare that had been his pasture mate for 16 of her 18 years. On normal days, that mare was marish (sometimes downright *itchy), but when he couldn't see she would lead him to the water trough, to the feeding stations, and back out to graze with his head laid across her back. So I guess, what I'm saying is that it can be done!