I've watched my sister-in-law deal with this in her mother. It's been very difficult for her. Since my mom died when I was 13, I think that the best way for me to relate would be to imagine how I'd feel if my husband became senile.
I have pointed out to my sis-in-law that although her mom doesn't know who she is now, there is still a spark of her essence remaining. You can hear it in her laugh and see it in the cute way that she winks at you. My dad was just beginning to enter that mentally fragile period when he died. But the last time that I visited him, I found myself taking on a new role around him. It wasn't that difficult for me, really, and he didn't mind as long as I didn't get too bossy. But it really depends on the level of senility. For the sake of our parents and their well-being, we just have to do it. My mother-in-law reached a stage when she couldn't do for herself any longer. Ironically, her daughter hasn't made the role transition very well, and now I'm in the role of mothering her mother. Or, perhaps, I should say that I have become a concerned friend.
I know that God will help you to do what you must. And I sincerely hope that your siblings are supportive, too. It's much harder if they aren't.