Question: Which of Robin Gibb's siblings were featured in Mythology?

Context paragraphs:
Lamplight: "Lamplight" is a song by the Bee Gees, released as the B-side of "First of May", but featured as the single's A-side in Germany.  It also featured on their double album "Odessa" in March 1969.  The song was written and composed by Barry, Robin & Maurice Gibb and featured lead vocals by Robin Gibb.  No other singles were released from the album, and the fact that the group's manager Robert Stigwood chose "First of May", which only featured Barry Gibb's voice for the A-side, that caused Robin to quit the group (he would return in 1970).

The Longest Night (song): "The Longest Night" is a ballad number performed by the Bee Gees, with lead vocals by Robin Gibb.  This song was written by Barry Gibb, Robin Gibb and Maurice Gibb, and was released in September 1987, from the album "E.S.P.".

Saved by the Bell (song): "Saved by the Bell" is a 1969 single written and recorded by Robin Gibb.  It was released in June 1969 and has been certified gold.  It was the lead single on Gibb's debut album "Robin's Reign", released in early 1970.  According to Vinyl Records, the song was co-produced by Kenny Clayton.  Gibb also made a promotional video for this song.  The song gained commercial success in Europe, but was a commercial failure in the US.

Mythology (Bee Gees album): Mythology is a box set compilation of recordings by the Gibb Brothers, mostly performed as the Bee Gees, arranged in a four disc set each highlighting a Gibb brother.  Barry and Robin chose their own songs (presumably their personal favourites), with Maurice's songs selected by his widow Yvonne and Andy's songs selected by his daughter Peta.  Several U.S. and U.K. hits are absent from this collection including "Lonely Days", "How Can You Mend a Broken Heart", "You Should Be Dancing", "Nights on Broadway", "World" and "One".

Robin Gibb (EP): Robin Gibb is the first EP released by British singer-songwriter Robin Gibb, released in 1985 on Amiga Records only in Germany.

I Laugh in Your Face: "I Laugh in Your Face" is a ballad performed by the Bee Gees, written by Barry, Robin & Maurice Gibb and released in March 1969 on the album "Odessa".  Most of the vocal solos are performed by Barry Gibb except for a little part in the middle of the song sung by Robin Gibb.

Tomorrow Tomorrow (Bee Gees song): "Tomorrow Tomorrow" is a song by the Bee Gees written by Barry and Maurice Gibb.  The song was originally intended to be recorded by Joe Cocker.  It was the first Bee Gees single released after Robin Gibb had quit the group which was now down to a trio featuring Barry Gibb, Maurice Gibb, and drummer Colin Petersen.

Robin Gibb: Robin Hugh Gibb (22 December 1949 – 20 May 2012) was a singer, songwriter and record producer, who gained worldwide fame as a member of the pop group the Bee Gees, with his brothers Barry and Maurice Gibb, his fraternal twin.  Robin Gibb also had his own successful solo career.

The Only Love: "The Only Love" is a ballad performed by the Bee Gees, It was written and produced by Barry Gibb, Robin Gibb and Maurice Gibb, lead vocals by Barry Gibb.  Released on their 1991 album "High Civilization".

Robin's Reign: Robin's Reign was the first solo album by singer Robin Gibb, a member of the Bee Gees with his brothers Barry and Maurice.  Robin had left the group following a disagreement with his brother Barry over who should sing lead vocals.  The album was not a commercial success, though it did spawn Gibb's solo hit, "Saved by the Bell" (UK #2).  The other songs in the album were produced by Gibb, and the rest was produced with his (then new) manager, Vic Lewis.  This album was reissued by RSO Records in 1978 and reissued in 1991 on Spectrum Records.  The album had a limited CD release in Germany and was made available digitally on Amazon and Spotify in 2011 and iTunes the following year.  Gibb would not release another solo album until 1983.

Supporting paragraphs: Mythology (Bee Gees album) Robin Gibb
Answer: Barry and Maurice Gibb