  We theoretically investigate the effects of charge order and spin frustration
on the spin ordering in TMTTF salts. Using first-principles band calculations,
we find that a diagonal inter-chain transfer integral $t_{q1}$, which causes
spin frustration between the inter-chain dimers in the dimer-Mott insulating
state, strongly depends on the choice of anion. Within the numerical Lanczos
exact diagonalization method, we show that the ferroelectric charge order
changes the role of $t_{q1}$ from the spin frustration to the enhancement of
the two-dimensionality in spin sector. The results indicate that $t_{q1}$
assists the cooperative behavior between charge order and antiferromagnetic
state observed in TMTTF$_2$SbF$_6$.
