Is a grandmother's breastfeeding of her grandchildren considered a forbidden act, given the different lineages of the other children she breastfed alongside them, and does the difference in the source of the milk (the grandmother's milk or her second husband's milk) alter the ruling on what is forbidden?

The rule is that any two individuals who have suckled from the same breast are siblings through suckling. Thus, if your suckling from your grandmother was such that it establishes prohibitory kinship, it makes you a brother to your mother through suckling, due to your shared suckling from your grandmother's breast. There is no difference whether the milk came from the first or second husband.

If the grandmother's milk was not nourishing or was scarce, the prohibitory kinship still spreads through it, unless it was not milk at all.

However, if the number of sucklings is unknown and there is no certainty that five or more nourishing sucklings occurred, then prohibitory kinship is not established.