I have heard that some (e.g. Chabad?) have the practice not to say kaddish for one's deceased sister. Can anybody confirm this? What might be the reason for this? I had thought there is an obligation to say kaddish for a sister for 30 days.
Kaddish is said for the deceased father or mother, regardless of how intimate or strained the relationship between deceased and bereaved. While the primary obligation is towards father and mother, it is also said, according to the custom of some communities, for other close relatives: brother, sister, son, daughter, and wife, for the 30-day period.
I'm not sure what communities have a custom not to say Kaddish for close relatives, but it appears that at least some don't.
Two reasons why the obligation of Kaddish is primarily for the parents:
Perhaps somebody is confusing this with the list of relatives for whom a kohein must become tamei for (Lev 21:2-3), which includes an unmarried sister (but not a married one). My Jewish Learning says that the rabbis took the kohein's list in torah -- father, mother, son, daughter, brother (including half-brother on father's side), unmarried sister -- as the basis for whom everyone mourns, and expanded it to include spouse, married sister, and other half-siblings. They cite Mo'ed Katan 20b. I just looked up that g'mara to confirm and also found there a discussion of second-degree close relatives (e.g. father-in-law); it appears there that the rabbis permit but do not require mourning in those cases, but I haven't chased it farther.