selina,
You can only claim grandparent / caregiver relief for ONE person, ie if claim on your father, then cannot claim for ah gong.
As for your other qn on your bro claiming parent relief on your dad and you claim grandparent relief, it's not clearly stated in the guidelines but i'm drawing conclusion based on the fact that a person can claim both parent relief and grandparent / caregiver relief because these 2 reliefs serve different objectives. Therefore, i think you can claim your dad for Grandparents / caregive relief for your dad.
In any case, it's ok to just claim for this one, if cannot, IRAS will just take out. You are not deliberately cheating them. They didn't explicitly say cannot claim mah.
weikuan,
selina still can claim for grandparents / caregiver relief for this YA cos C not in infantcare yet.