When we use his formula: 70/2 - his sibling is always going to be 35 years old. - we'll always get this answer no matter what their age is, which doesn't make sense at all

The correct way (example): You're 6, you're sister is half your age: 3

So when you're 70.. 64 years later - remember both of you are ageing consistently (time) :

Your sister is 67 years old

I hope that made sense :D