What on earth do I need a study for? Read any society magazine, Tatler is good, or read the newspapers. I have plenty anecdotes though, including my first cousin by marriage (m to my cousin) who was given a flat by her banker CEO father at 19 years old. It was considered a starter flat. And it was expected by the way. And nothing thought of it. She and my cousin now own 3 properties the last one in the million plus, ballpark. Imagine how many she'd own today without that initial help? She earns well, so she'd own, but she'd not own three. I suspect at least one will be given to their two children or they will both most certainly receive deposits. She would find this attitude of not supporting your children with your assets strange.
They may not give their children everything, but as a rule, the parents of the extremely wealthy don't see virtue or capital in letting their children really struggle. It's not how wealth is fostered in the family. They know how money works, the belief is that money should be put to use, so it can grow more money. If you give your children the ability to get on the property ladder early its an investment, property will only get more expensive, they will only have to borrow a larger sum later, plus, the flat can be sold later at a profit. It frees up the children to invest their money into something else. Often some of the wealth is transferred relatively early with some held back, kept in trust etc. It's all about growing family wealth. Clearly the OPs in-laws have that view having helped out her sister.
This idea of granny and grandad/parents sitting on a stagnant nest egg that never hatches, while the children cannot afford to save, have children, or put surplus money to use is alien to them. And they don't treat their children like greedy mercenary bastards for expecting some initial investment either.