[ My uber-Dem sister in law is the same way. It’s a way of emphasizing importance and disrespecting those with whom you have an appointment. ]
Invite her over a few times for dinner and on the 3rd or 4th time she is intentionally late tell her go home when she arrives and that the disrespect wuill no longer be tolerated..
Sometimes we eat most of the food and tell her it was getting cold. It seems to have helped a little.
Also, our kids used to joke about it in front of her.