I can't see if the OP has replied to their initial message as too many pages of messages! But the older generation do say life is more expensive today, seperate to relative change.
If I had excess money I would definitely help my children get onto the property ladder, renting is money in the home owners pocket and if it's possible to buy then it's an investment for life. If there's excess money then this can be taken off any inheritance they could have received upon their death. I don't see the point in waiting, by the time the older generation pass now, the adult children are normally financially stable and not in need of the money. It's so much harder to get onto the property ladder nowadays. Also knowing the OP sister has been helped by her in laws will be hard to muster even though her own parents didn't help her.
Basically I don't see parents supporting their adult children as money grabbing at all, if they have it. My children will always be my children no matter what age, and if I can help them financially at 30,40,50 I will. They will of course not expect this and be grateful.
My husband and I had children before we bought and have had to struggle to get by at times and get onto the property ladder. Literally surviving on £25 a month some months after paying the bills, but that's life at times. But my FIL did help with a deposit for our home because he could afford it, and this was after us paying him rent on a second house for 3 years which he bought where he gained £80k in equity.
So if your parents can afford to help you as a one off to get onto the property ladder I don't see this as ungrateful or entitled. The children as also their grandchildren which I will one day see as an absolute blessing for me to become a grandmother. If my children are hard working, earning their own money and not blowing what they have on unnecessary items then yes I'd help them