。
I have a large family to support.
Literal
Support-[and-て] must-[ねばならない] [attributive-の] large family [subject-が] exist-[explanatory-のです].
養う ('to support / raise / feed [a family]') + ~ていく (ongoing future 'continue to') + ねばならない (formal obligation) = 'must continue to support.' This all-one-verb-phrase modifies 大家族 ('large family') as a relative clause. Very compact; English needs 'a large family that I must support.'