not sure I agree with this.
<root>
<base1>
<something1>
<base2>
<something2>
<something3>
<base3>
In the above, the siblings are: base1, base2, base3 …. also, something2 & something 3 are siblings.
Base1 & Something1 are parent/child. so is Base2 & Something2.
Think of a sibling as "brother/sister"… not parent/child.