That is only assuming you retain your current lifestyle, but if your parents had 10mil, they would be living a more luxurious lifestyle.
Don't be surprised that a 10mil net worth family's monthly expenses are around 20-30k a month, even after excluding house mortgage (likely paid of).

You may find it hard to believe the expenses are that high, but imagine your monthly expenses, people who are earning 1.5k household income find that ridiculously high also.
At 10mil, if your parents retire, they have to be quite conservative with that money, firstly, net worth includes property, and your house is probably at least 2mil if not 3. So you only have 8mil.
8 mil at 3% per annum roughly enough to cover monthly expenses, but you have to factor in inflation, so you need to invest the money to a point where the money still grows at a reasonable rate to hedge inflation while still covering your monthly expenses.

If you and your sister both want to rely on that 10mil as well, and you and your sis have families, and also want to live the same lifestyle, that is 20k a month extra for you and another 20k extra a month for your sis.
So it's not straight forward easy like you assume.

If your parents suddenly win 10mil from lottery today, then that's a different story, but it's not easy to suddenly down grade your lifestyle from a family worth 10mil to the lifestyle of a family worth maybe 1mil.