A certain section of the Brahmakumaris don't use the label Hindu, just like the Lingayats, do to distinguish themselves from certain features of Hinduism like casteism, untouchability, animal sacrifices and so on.
I have met many brahmakumaris who use the term Hindu as well. It is a matter of choice.
The term ' Hindu ' is just a superficial label coined by foreigners, which covers many diverse sects and groups, whose beliefs differ from each other. It is used for identification purposes alone and nothing else.
It is conscious action, living and virtuous conduct that is the basis of dharma and not blind adherence to superficial labels.