to think that if you ask to borrow someone's car, you should be willing to meet the cost?

[TidyDancer]

It's not me, it's my sister.

She has kindly agreed to our brother using her car while she is out of the country. It will be for a period of about a month I gather.

As our DB is a younger and more inexperienced driver, it will apparently cost a bit to have him on the insurance. DB has not made any attempts to offer to reimburse DSis for the cost of this, and keeps making comments that seem (at least to me) obvious he is expecting DSis to meet the additional cost. He is acting like he is doing her a favour by taking care of the car while she is away.

IMO he should be paying the additional cost without a second thought. I don't know how much money we are talking about here btw, but it's the principal of it I think.

AIBU?

I wouldn't lend anyone my car if they were quibbling about paying, you just know where this will end up.

When DH's sister has been home from NZ DH puts her name on our insurance policy for a set time.
There is an fee to pay but it has only been something like 50 quid.

I agree with others - he should get his own short term insurance. If he prangs it, your sister will end up paying the excess on her insurance policy and increased insurance quotes for years to come.

My sister lends and borrows cars like this all the time and it gives me the wibbles. I need my car, and if I thought my premiums had gone up because of another driver, I'd seethe while smiling politely. Not worth it IMO while temporary insurance is available.