Sentence:  Finally , whatever the techniques used , a twin goal <WSD>is</WSD> common to all preventive casework service : to cushion or reduce the force of the stress impact while at the same time to encourage and support family members to mobilize and use their ego capacities . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)