Unfortunately, not everything is within our control. Having two adult children who fight constantly is not doubt distressing for you, and whilst you may try to mediate the situation for a while or make things better, ultimatley this is beyond your control and it is up to them to work it out.
What you can control however, is the level of distress it is causing you. You can let them know how much is distresses you and that if they are going to continue fighting it is not something that you want to be exposed to or drawn into. You can suggest they seek professional help for mediation but ultimately this is their decision, and as two adults they need to take responsibility for this.
You need to put your energies into you and looking after yourself. I know it is hard and you hope and wish for a family where everyone gets on, however, there is no point invisting your energy and emotion into this when you cannot do anything about their behaviour.
I would begin by having the conversation ith each of them and maybe involve your husband and let them know the impact it is having on you and your relationship with your husband. If they are going to continue and not make any changes as a result, then you ask that they respect your needs and ensure that you are not involved in these discussions.
You need to focus on what you can do something about. In this instance you cannot control or make things better for them (they need to work this out) but you can control the impact it is having on you to some degree by expressing yourself to them and asking them to respect your needs and what that involves.
Best wishes Ruth, and remember, focus on what you can control - otherwise you will only end up feeling depleted, furstrated and sad.
Take care of yourself,