Big brother, you have a terrible fever! Never mind the bags, just rest in the shade of that tree!
Literal
Big-brother, amazing fever [copula-だよ]!? Luggage [topic-は] fine [because-から], that tree-shade [at-で] resting [progressive-てて]!
すごい熱 'amazing/terrible fever' — すごい here intensifies the severity. 荷物はいいから 'forget about the luggage (it's fine)' uses いい in its dismissive sense. 木陰で 'in the shade of a tree' (木 'tree' + 陰 'shade'). 休んでて is the casual contracted progressive command (~ていて → ~てて) — 'be resting!' Urgent, caring sibling speech.