Kukaa can be used with different meanings: a. be seated, as in amekaa, he is sitting. If you are referring to present time you must use the -me tense because kukaa, with this meaning, is one of the verbs of state like kulala, etc. wanakaa = they are in the process of sitting wamekaa = they have finished the process of sitting, i.e. they are sitting. b. stay or live somewhere as in nitakaa Mtwara. In the present tense you would use -na-: Wanakaa Mombasa, they are living in Mombasa
I think "wanaketi" should be accepted as an alternative to "wanakaa".
keti kt [sie] sit. Source: TUKI KAMUSI YA KISWAHILI-KIINGEREZA Swahili-ENGLISH dictionary. http://www.elimuyetu.co.tz/subjects/arts/swa-eng/k.html