Besides a physical check up and looking at nutrition you can also investigate your relationship with your bird and how you interact with her/them.
Birds live in flocks and will often have an established hierarchy. Some birds will be more dominant than others -- and the same will happen in the home environment. You and whoever else interacts with the birds are part of the flock and thus part of the hierarchy whether you like it or not. Our parrots also display this behaviour and as long as you can show that you are the 'alpha' they won't come and bite/attack you because they know it's not their place to do so. Here what goes on inside you and how you experience yourself towards them plays a big role -- they know how you feel and if you act one way but feel another, where you try to be assertive but you're actually quite fearful or not really wanting to be in a position to tell your bird to stop it -- they will pick up on that and know that you're not being genuine and flatout ignore what you are doing, because they can see behind the mask and continue what they were doing.
Many people find it hard to place themselves in a 'dominant' / 'directive' position because they believe that they are somehow infringing on the 'nature' of the animal/bird and believe that they are on no position to intervene and believe that the right thing to do is to just 'let them be'. What you get then is spoiled animals with no direction which are like demon children out to get you and make your life miserable -- because you won't stand up and say no. Animals, just like humans, require guidelines and principles that you have to set out in terms of what is acceptable and what is not. If you 'let your bird be' then the bird interprets this as 'anything goes' and will behave accordingly.
With our Parrots I've learnt to read their body language by observing them and can pick up on language that they read as 'dominant'/ 'I'm in charge'-- they will for instance want to be on the highest perch above everyone else or make themselves taller next to another mate and bicker with them until they lower their head into submission. So we use our fist and pretend it to be our head and will make sure it towers over whoever is trying to dominate/bully another parrot until he stops and drops his head. Sometimes they will bite and if you then back off and become fearful, they see at as a 'score' / +1 for them and again not take you serious. So even when they bite, don't react emotionally within backing off or the opposite where you hurt the bird in anger to make a point. You simply go NO and don't react. We had a parrot like that and he bit me a few times but I didn't let it get to me. I used to go 'AW!!' when he would bite because he has a very stron bite and sharp beak -- then I stopped doing that and the bird actually got a fright, stopped biting me and kind of looked at me sideways and went 'Aw?'-- like 'why are you not reacting/squeeking' like I had become his personal squeeking toy for his entertainment.
Any form of anticipation or expectation that your bird will be aggressive will also only perpetuate the problem. Whenever you deal with your birds, before hand take a breath, stabilise yourself and make sure you are 'empty' in terms of no emotions and back chat -- because again the bird will pick up on it and then be agressive, which you as the human can then interpet as 'see, she's doing it again -- it's her, not me that is the problem' which then again only feeds/confirms the pattern.
So investigate your approach/relationship towards your birds -- who are you in relation to them, what do you stand for, what will you accept and allow. Birds are very sensitive and everytime you participate with them you really have to be here in full awareness with them. If you find that other people like your parents are not effective with handling the birds / not changing their approach, it may be better for them to not interact as it will be both unpleasant for them and the bird. So I suggest you use this as an opportunity to do Self Forgiveness on any fears, beliefs, emotions and back chat you may have in relation to your bird and establish a new relationship that consist of mutual respect and consideration. Birds are not the only one's in this -- the same approach hold for dogs and horses for instance as well.