Is this your? 可以吗

Is this your? 可以吗 Is this your sister?英语语法Is this your sister?No,it isn't.Is that your brother?Yes,it is.为什么这里的“your sister”、“your brother”在回答时要用“it”代替?可以用“she”或“he”代替吗? Is this your sister?可以这么回答吗?No,she isn't.但是课本上说的是：Is this your sister?No,it isn't. Is ths your bag?“怎么回答?可以用This is my bag吗?为什么?我主要是问可以用This is my bag吗？ Is this your cousin 中有错吗 is this your pen blue可以这样问吗你的笔是蓝色的吗 is this your skirt this is your Maybe this is your Is this your uncle? this is your aunt. is this your business Is this your schoolbag? Is this your (空格) Is this your schoolbag? this is your pencil Is this your cake. This is your explanation?