Is it permissible for these two boys — who are unrelated to the questioner except that they are the sons of her grandmother’s husband — to see her without hijab and sit with her? And are they considered to be in the position of maternal uncles to her, which would make it permissible for them to see her?

If your grandmother married another man who is not your mother's father and bore children with him, they are your mahārim (unmarriageable kin), specifically your maternal uncles (akhwāl), because they are your mother's brothers from her mother. A maternal uncle is considered mahram (definition). It is permissible for them to see you without a hijab (definition), based on the Almighty's saying: "Forbidden to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother's daughters, and your sister's daughters" (An-Nisa: 23). And you are their sister's daughter. There is no difference in this regard whether they are older or younger than you.