Problem witn select

I need the addresses from a database. The persons belong to different groups. The groups are identified by IS_ID. Some groups have to be joined though. For instance groups 2 and 3 should be joined. Tried to accomplish this using the following
SELECT
PERSONEEL.PS_NAAM,
PERSONEEL.PS_VOORNAAM,
PERSONEELADRES.PA_STRAAT,
PERSONEELADRES.PA_STRAATNR,
PERSONEELADRES.PA_STRAATBUS,
GEMEENTE.GM_POSTCODE,
GEMEENTE.GM_DEELGEMEENTE
FROM
PERSONEEL,PERSONEELADRES,GEMEENTE,DIENSTVERBAND,INSTELLING,SCHOOL
WHERE
PERSONEELADRES.PA_PERSONEEL_FK = PERSONEEL.PS_ID
AND PERSONEEL.PS_ID = DIENSTVERBAND.DV_PERSONEEL_FK
AND PERSONEELADRES.PA_GEMEENTE_FK = GEMEENTE.GM_ID
AND DIENSTVERBAND.DV_SCHOOL_FK = SCHOOL.SC_ID
AND SCHOOL.SC_ID = INSTELLING.IS_SCHOOL_FK
AND IS_ID IN (
SELECT
CASE
WHEN IS_ID = 2 THEN '2,3'
WHEN IS_ID = 3 THEN '2,3'
WHEN IS_ID = 4 THEN '4'
WHEN IS_ID = 5 THEN '5'
WHEN IS_ID = 7 THEN '7'
WHEN IS_ID = 10 THEN '10,11'
WHEN IS_ID = 11 THEN '10,11'
WHEN IS_ID = 12 THEN '12'
WHEN IS_ID = 14 THEN '14,15'
WHEN IS_ID = 15 THEN '14,15'
WHEN IS_ID = 17 THEN '17,18'
WHEN IS_ID = 18 THEN '17,18'
WHEN IS_ID = 21 THEN '21,22,23'
WHEN IS_ID = 22 THEN '21,22,23'
WHEN IS_ID = 23 THEN '21,22,23'
END)
In the final version IS_ID before the In has to be replaced with a parameter.
Now I only get the single values, ie 4, 5 and 7.
Can anyone help me to get this right.
Many thanks in advance.

Luc Denys