All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The money of Zakah is fully owned by the person to whom it is given once he receives it and the rulings concerning private wealth apply to it. Hence, there is no sin on the part of a well-off person to eat from that which is obtained with Zakah money if this is done by its poor owner's permission.
So, as you participate in the expenditures of that family, there is no wrong in you eating from that which is obtained from the Zakah money that this family consumes. There is also no wrong to live with your wife with them and share their expenditures and livelihood.
Please note that your intention to marry one of the daughters of this family does not make it lawful for you to intermix with them or sit in seclusion with any or some of them.
Allaah Knows best.