"She has four older sisters."
It's not right.
口 is used for counting the number of people in small groups, like a family, a clan or a village. People in these groups are supposed to rely on each other for livelihood. And "sisters" do not form such a group.
This quantifier gives a feeling like "well you see, I've got these mouths to feed". But it is not considered vulgar or overly colloquial. You can use this quantifier in formal texts.
Um, In modern Chinese (and many other similar languages in East Asia), you need a "measure word" after the number when you count objects.
The requirement of "measure word" is due to the fact that there is no "countable" and "uncountable" nouns in Chinese. A countable noun has a default unit of measurement. When you say "four sisters", you are actually counting the "number of identities" of sisters. And you can't say "four water" because there isn't a default way to measure the amount of water.
But in Chinese, you need to declare the measurement unit explicitly:
He has four (identities of) sisters.
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).
个 is not a number. 个 is called a "measure word" or "classifier" (depending on which textbook you read). You need a meaure word any time you are counting things, and any time you say this/that/these/those things.
A measure word lets you know the kinds of things you are counting.
We don't have many measure words in English, but we do have a few. Consider the difference between: Four loaves of bread Four slices of bread Four kinds of bread
In Chinese there are measure words for almost everything! 个 is the most common measure word, and is used for things that don't have their own special measure words.