First of all - every bird is different.
There's no "one right way" to deal with overbonding - it depends on how the behavior got that way in the first place.
Your conure is the right age for sexual maturity - and if it was handfed, that means it looks to humans to find a mate. It's natural for it to pick one member of its "flock" and defend that person aggressively against all intrusion for a long period of time.
However, that person, to stop it, needs to wholly 100% reject the conure. I used to read posts where the "mated person" would laugh or encourage the behavior. That seals the deal. Conures, like all parrots, can read human emotion, expression, behavior, and body language. From now on, from the moment you next see your parrot until the biting stops, your mom cannot hand the parrot at all.
I'm not sure who invented that 90%/10% rule - but it didn't come from any scientific magazine i ever read.
Bonding rejection in the wild happens in one way. Anytime a potential mate would approach, a rejection would be a simple snap. No pain, no nothing, just a snap and fly away. In other words, no touch, no sound, no contact. Your mother cannot ever be in the room when you are in the room with the bird - because what will happen is the bird will attack YOU in order to show your mother that it is strong and aggressive. It's not actually punishing you, it's trying to show your mother that it's a good mate.
By ignoring the bird and never going near it, your mother is showing the conure that she's not interested in the slightest. This may take weeks. I've heard it taking months.
100% of the time - the reason birds STAY OVERBONDED is because the overbonded person rejects this advice, is too attached to their parrot, or thinks the overbonding is funny or "cute" and cannot leave their parrot for long enough. They still talk or interact with their parrot, and every time they interact, the bond is maintained.
You must be the only one (and anyone else in the house, just not your mother) to interact with the parrot. If he bites, use a dowel. Take him in and out of the cage. Feed and change his water. Change his papers. Do everything normal, talk to him, interact with him. Act as if nothing is out of the ordinary. If he acts aggressive - just keep away from his beak and don't let him bite you - never let a bite happen. Never use gloves to pick him up, because eventually the gloves have to come off anyways and he'll just bite when he sees you're not wearing them (parrots aren't stupid, we just assume they are).
Eventually, a very painfully long time from now, he will give up. If you're will to brave it, it will go faster to try and pick him up through the biting. Show him that no matter what, your hand isn't going away despite the biting - you won't lose a finger. You're above him in the flock. If you'd like to wait longer just offer your finger, and if he looks like he'll bite, substitute for the dowel. If he bites the dowel, he's not ready. Keep doing it - eventually he'll go back to normal. Never ever ever give him what he wants - to be with your mother.
If your mother says this is silly - explain to her that overbonding is dangerous at home. In the wild, it leads to breeding, which is fine, but at home, the behaviors associated with breeding can be dangerous. Over a long period of time, a parrot might stop eating unless your mother is present, because it will save it's food to regurgitate in her presence. If she stops visiting the parrot, it will stop eating - and may get very sick or die. It can also have severe behavioral problems if it can't be with her all the time (especially if its wings are clipped and can't fly to her to be with her).
Good luck! Dealing with overbonding is never fun - and make sure it doesn't happen anymore. Be a flock family - have everyone handle your bird all the time and always treat it well. Never do things like pet it's rear hindquarters or feed it food from your mouth or let it put food in your mouth (those are what start overbonding).