My ex just died and he was single. His sister and mother are trying to keep all of his belongings .

If there is nothing of real value, it may not be effective to open an "intestate" estate (without a will) except to extinguish creditors. His children should be first in line to inherit his belongings. Please consult a probate attorney.

Yes, his children are entitled to his belongings (even if it's only a broken down motorcycle). Go to the DMV for instructions on how to transfer the motorcycle. Someone (child or guardian?) may need to file a small estate affidavit.

As a fellow Harley rider, even a broken down Harley is a thing of value in the eyes of the beholder. The Texas Estates Code spells out how an individual's assets will pass if the individual dies without a Will. Go see an attorney who will explain this to you and can communicate with the sister / mother.

Yes, his children should inherit. Hire a probate lawyer on their behalf.