Grandfather is جَدّ (jadd, pl. أجداد ajdād), and grandmother is جَدّة (jaddah, pl. جَدّات jaddāt).
A number of languages distinguish between maternal and paternal aunts and uncles; English is not one of them, but Arabic is. A paternal uncle is عَمّ (ʿamm, pl. أعمام aʿmām), and a paternal aunt is the feminine form, عَمّة (ʿammah pl. عَمّات ʿammāt). Maternal uncle is خال (khāl, pl. أخوال akhwāl), and maternal aunt is خالة (khālah, pl. خالات khālāt).
One thing to bear in mind is that we’re talking about blood relatives only; in-laws are another entry.