I looked in the GRO deaths index, and I found only 5 entries for Bilibin. I am guessing that Alexander is the other son and Gwendoline his wife.
Mary E V Bilibin d. Q2 1962 aged 88 Henley
Alexander Bilibin (b. 2 Feb 1903) d. 1972 Chichester
Gwendoline Bilibin (b. 13 Mar1904) d. Jan 1990 Chichester
Ivan Bilibin (b. 26 Dec 1908) d. Aug 1993 Reading and Wokingham
Jean Mary Bilibin (b. 10 Aug 1915) d. Aug 2000 Henley
So from what you tell us, it seems quite possible that Mary, the mother, lived with Ivan (or nearby). His address was the same from 1962 up to 1984: Maitlands, Wood Lane, Sonning Common. (Kidmore End was the exchange.) So it's probably
Sonning Common that you should look at (apologies, I didn't have time before to look at the full directory entry, just the index). Perhaps Ivan was buried near his mother.
Sonning Common is also in Oxfordshire, and is not the Sonning in Berkshire.