My love, if she's being abused by him in any way, he CAN'T attend counselling. He'd use it to abuse her further.
She will leave when and if she realises that she has to. Nothing you or anyone else can do, other than to reassure her that you will all be there for her, and that HE is not treating her at all well. You can tell her that the DD IS suffering in this environment, and that it's teaching her that this is a normal relationship, when it is anything but.
On average a woman is assaulted 35 times before seeking help.
Every 3 days a woman is killed by her partner
A victim of domestic violence take on average 7 attempts to leavee before they actually do.
this situation is far more complex and devastating to your sister than I think you appreciate/understand.
You need to keep reinforcing the message that the man can change in a heartbeat if he wants to, but he doesn't. She needs to read WHY DOES HE DO THAT by Lundy Bancroft if it's domestic abuse.
Whatever it takes to get her away from a man that abuses her. That's what's needed. If your Dsis is having counselling, at least she has one form of appropriate support and can offload the worst to the therapist and not her family.
Unless your parents REALLY are frail and are feeling a strain, then tbh, what else would you have them do? Turn their backs on her? Mine did and my darling sister too. I'll never forgive them. It could very well have cost me my life. They knew this too.
So if you are merely supposing that they are feeling the strain of doing this, rather than knowing it as a fact, then you are in danger of coming across as resentful of the SHEDLOADS of help your sister's very LIFE may depend on. Your niece too.
This is a long road. You may be bored of it, but I'm guessing your sister is suffering more than you are. If you can, help her. If you can't, don't ever begrudge that help.