Ariane - I too have been guilty of screaming at my ex that he has no right to complain as it is me having to deal with the epilepsy, Hypermobility & associated dislocations and arthritis. 
It's NOT nice to do that, but sometimes as the ill one, if your disabilities are at a really bad point, it CAN rule your life, and you feel aggrieved at someone healthy complaining at you.
I DO apologise when that happens, but if your mum is finding it hard, then SHE needs to speak to someone outside the situation (NOT your sister OR you) to offload the stress. Maybe a counsellor. There ARE counsellors out there that specialise in helping Carers.
It's unfair to offload the stress of Caring for someone ON the person with the disability.
In that case, I WOULD and DO feel for your sister and see her side - she is being had a go at for something that she never asked for, doesn't want, and fucks up HER life.
(I KNOW it's unfair to yell back at the people Caring for you when they are offloading their stress on you, but equally it's unfair TO offload that stress on the person with the disability)
The time REALLY has come where your family needs a lot more outside involvement.
Your mum needs a Counsellor to offload the stresses if Caring got your sister.
Either your mum or sister's boyfriend NEED to give up work, or both work PT and care for your sister PT. If not, then your sister WILL just have to accept an outside Carer.
Your sister needs to think long and hard about TTC without FAR better seizure control. Uncontrolled epilepsy is just as dangerous for an unborn baby as Epilim / sodium valproate. If the seizure is bad, the baby could die.
Your sister AND your mum MUST be made to understand that due to your family's own disabilities, you are UNABLE to provide care to your sister. AND that it is NOT your responsibility to do so - you did not choose to give birth to her, and you did not choose to be in a relationship with her.
You yourself NEED some outside help. You are dealing with multiple disabilities in every member of your family, and due to your own disability you need painkillers to get through the day.
You NEED to take a step back - either take the DC's to school by bus, or let your DH drop them off. That way you aren't making yourself do reliant on your mum, and YOU are taking back CONTROL of your OWN life.
Please listen to me, your DC's need YOU to be well enough to take care if them, and you WON'T BE if this situation is allowed to continue.