Before answering this question, please read the following story which is based on a real case.

Mrs. X died intestate. A few days later, her mother Mrs. Y died, too. Mr. X and two children inherited Mrs. X's estate.

One month later, Mr. Z, Mrs. X's brother and Mrs. Y's only son, claimed a portion of Mrs. X's estate. Mr. X was shocked and believed that the estate had nothing to do with Mr. Z. So he consulted a lawyer, but was unexpectedly told that Mr. Z was entitled to 1/4 of the estate.

According to The Civil Code of the People's Republic of China, dated Jan 1, 2021, heirs are classified into two lines:
First in line: spouse, children, and parents; and
Second in line: siblings, grandparents, and grandparents-in-law.

When Mrs. X's died, four persons were entitled to succeed to her estate: Mr. X, children, and Mrs. Y, who were heirs first in line. And each of them was entitled to 1/4 of the estate if no other factors were considered. Please note that Mrs. Y was still living when her daughter Mrs. X died.

When Mrs. Y died later, her estate came to the hands of her son Mr. Z, inlcuding the portion inherited from Mrs. X. Therefor, Mr. Z had a lawful claim for 1/4 of her sister's estate.

From the above analysis, it is not difficult to reach a conclusion that a last will is necessary for anyone to dispose of their estate as they wish. Otherwise, their estate will fall into the hands of others even if it is against their will.