On Wikipedia's Related Key attacks page, there is a section about WEP as an example to related key attacks.
Encryption uses the RC4 algorithm, a stream cipher. It is essential that the same key never be used twice with a stream cipher. To prevent this from happening, WEP includes a 24-bit initialization vector (IV) in each message packet. The RC4 key for that packet is the IV concatenated with the WEP key. WEP keys have to be changed manually and this typically happens infrequently. An attacker therefore can assume that all the keys used to encrypt packets share a single WEP key. This fact opened up WEP to a series of attacks which proved devastating. The simplest to understand uses the fact that the 24-bit IV only allows a little under 17 million possibilities. Because of the birthday paradox, it is likely that for every 4096 packets, two will share the same IV and hence the same RC4 key, allowing the packets to be attacked. [Bolds are mine]
RC4 accepts 40–2048 bits key sizes. The WEP designer selected some parts of the key as an IV to the input for RC4.
Actually, this attack purely uses the (IV,key) pair reuse. The attacker just waits for the collision to attack. This is common in any stream cipher.
What does make this a related key attack? In a related-key attack, don't we combine some distinct but related keys to attack the cipher?