not a tax expert here,...so may i just kay poh abit here

according to LHDN "with effect year 2015 an individual who earns an annual employment income of RM34,000 (after EPF deduction) has to register a tax file."

thus yr sister should just go register for a tax file,....as to whether she is subjected to any tax able income,...it has to depends on the actual income and the total cumulative tax relief she is eligible to claim

if the agent said its best to register in Dec & pay for 2021's dues in 2022. ....then i am sure i am no smarter than that tax agent....

but i think if she is subjected to tax for her income in 2021 that are declared in April 2022,...then i think i read it before,...LHDN may ask you sister to submit tax declaration for year of income in 2020, 2019, 2018 or until the year she is not subjected to taxable income.