You wouldn’t believe the number of false victim cases, where women claim to be beaten up just so they can get money/kids in a divorce or keep thier husbands under their control. Domestic violence is a true problem, and I know that like 90% of cases are not lies, but there are sly women out there to take advantage of it, esp. in the west where women are given the benefit of the doubt.
However, I do not think your sister is lying because she doesn’t seem to want anything out of it. And the fact she doesn’t want to tell ur dad is that she’s afraid the situation will get out of hand. maybe she has a diff definition of “beating up”, like a violent push, but he should not be inflicting ANY violence with her- it’s a slippery slope. Does her husband seem like the type that gets angry and violent or is he the subdued type? Is he the type that would have affair (i.e. not very religious?). Only you can answer these questions. I think your sis is telling the truth, but she needs to sort it out herself. Tell her to call your fam if anything serious happens and not to hide it. Be a good listener and support her.