"She has four older sisters."
is it not more accurate to use 四位 instead of 四个 since we are talking about humans here?
Both is valid, but 四位 would be a bit strange here.
Just as you know, 位 is a more polite measure word for human, but the politeness is not required everywhere. For family members, 位 is commonly used for elders in formal occasions.
我曾经有一位母亲。In the past I had a mother.
Note I use 母亲 instead of 妈妈 to add some solemnity.
But for people in the same generation, this becomes a bit weird:
我有一位弟弟。I have a younger brother.
Acceptable, but uncommon. If the family member is younger, it could be hilarious:
我有一位孩子。I have a child.
Besides, using 个 instead of 位 would not be considered as disrespectful except in some idioms like 各位 (ladies and gentlemen).