Aoife wants Brid to take her place in the struggle. Enda was no use - he wanted to get married and move to Donegal - away from the strife (both internal and external). Ina was also not interested in the fight.
Aoife, when confronted, says she couldn't continue the fight herself because Sean wouldn't have it and she was afraid to challenge him (so resorted to belittling him instead.) Tad, also was useless. So it fell to Brid.
Aoife accepted that Ina was above the fray and demanded that Brid protect her. When Ina was killed, she blamed Brid - "It should have been you," meaning not only that she couldn't forgive her for not protecting Ina, but also that - as the designated IRA fighter, she should have continued until she was killed herself. Even without Ina's death, she owed that to her mother and to the oppressed people of Northern Ireland.
Very sad.