my friend complain of his mother beating his elder sister of about 26 years old anytime she make a mistake and her sister did not want to reply her because she is respectful.

No vex o, How would she have replied? Beat her mother back?  Let your friend tell you the full story. There is also no law of the land which states that an unmarried lady must be in her parents house. Above all, dialogue resolves most issues. Both of them should try to communicate with their mother.

at 26 ur mama still dey beat u, i swear u be the heaviest mumu ,wen u no be beast of burden aka maluuuuuuuu, or maybe the mama don beat am damage her brain,
kai, naija is wonderful ooooooo no offence to my mama but i swear them no born am well to beat me at age 20 talk less of 26
talking as a child whose mum believed in flogging or beating children, it effing sucks, i lft nigeria aged 17, i think say my mama still dey beat me till i was 17 sef,,,,but i just pam because i know say na interim thing, i comot naija, 4 years later my mama come england come stay 4 my house, somthing happened my mama wan slap me, oboy i see purple, i hold am for hand i say mama, mama, mama. mama hw many times i call u, she say 4
i say if u ever pull that stunt again, u go seriously wonder who born who

Most people at 26 are in their matrimonial homes not their parents. So maybe it's time to move away from home. That notwithstanding I don't think it's right for any mum to hit anybody at all not to talk of a 26yr old. She's an adult.