Thailand has 6 levels compulsory heirs called 'legitimate heir' from order of priority
1 child and Spouse
2 parents
3 siblings
4 half siblings
5 grand parents
6 uncle, aunts,
the next level don't get their share until the above level has their share, so your sibling don't get your estate unless you don't have a surviving parent or child/spouse
even if you put the grandchild in the will, the other in the legitimate heirs list can contest the will in court, eg, if you put the grandchild as the only receiver, the other relatives could contest this in court