What have you tried so far? Do you know their first names? Was in fact your grandmother a Holocaust survivor?
If you don't know their first names, and your grandmother was a Holocaust survivor, I would start in three ways:
- Look at the Yad Vashem Pages of Testimony web site. You can do an advanced search and search for your grandmother as a testimony submitter. Possibly, she submitted pages of testimony on her siblings as well as other family members.
- Look at jewishgen.org. By searching for your grandmother or, ideally, your great-grandparents if you know their names, you might have a shot of finding the siblings' names as well.
- Try and find out if your grandmother did a testimony video for the Shoah Foundation. She may have and mentioned them.
If your grandmother was not a survivor, in addition to searching at jewishgen.org, I would recommend you take a DNA test if you haven't done so. You may be able to connect with cousins who descendants survived and have built a family tree that includes your grandmother's side.
Once you have the siblings' names, in addition to Yad Vashem, I'd conduct searches at the United States Holocaust Museum web site. They have a significant amount of information that might provide clues.