My understanding is that the largest index funds use free-float weighting. This adjusts the weight of a stock in the index by the proportion of the stock that trades freely in the market. https://en.wikipedia.org/wiki/Capitalization-weighted_index#...

In the example you give, this should result in Berkshire Hathaway Class A to have a very small weight in an index fund weighted this way.

Looking at the holdings of VOO, as of July 31, 2019, It holds 806 shares of Berkshire Hathaway (Class A) (worth ~$249 million), but about 37 million shares of Class B, worth ~$7.6 billion.

https://investor.vanguard.com/etf/profile/portfolio/VOO/port...