If I open an account at a forex dealer such as Oanda, and my brother similarly opens the same type of account at the same forex dealer, would the forex dealer give both of us the same bid/ask prices? Or does the forex dealer give better/worse prices to different clients?
Probably. But not necessarily same executions. Maybe things have changed these days but in years past FX brokers would take the other side of losing traders trades, who get a very quick fill. But a trader with a winning record might have the order sent to the market, get a slower fill, or outright rejection if the market moved etc.
There are dealers today who still take the other side of traders and depending on which book the trader belongs to according to the broker, the order will be executed differently.
You can check yourself with two different accounts and then login same time but with two different IP address (landline + VPN or mobile or something else) and two different devices (so PC desktop + notebook or mobile phone). Then can you compare the quotes. If they are not the same then you know the answer. It may be also broker dependent.
I think prices may vary slightly between brokers due to different liquidity providers and different spreads. Also, during periods of high volatility, prices may change quickly and different brokers may update their quotes at different speeds.