What about having her mom walk her down the aisle. Alternatively, she can walk alone or you can walk together. As for a way to honor her grandfather, I would get a bouquet charm with a picture of him/the two of them. That way he can walk her down the aisle regardless of who walks with her.
Your fiancee doesn't have to be walked down the aisle by a man or at all. Here are some options:
-She walks down on her own
-You walk with her
-Her mom walks with her
-Both her mom and dad walk with her? If she wants to have her dad involved in some way, this could be a good compromise.
Also, I've seen brides at weddings attach something to their bouquet that belonged to a deceased family member, like a piece of jewelry or a photo. If your fiancee has something of her grandfather's, that could be a way to honor him and have him there while she walks down the aisle.
She could walk herself down the aisle. That way she wouldn’t have to let her dad do it but she also wouldn’t pick anyone else
You mentioned she had siblings. Is she really close with one and she would like to share her moment with? Does she have a brother?
I never had a close relationship with my father until later in my life, my mom was always my rock growing up. I decided for my wedding to have both my mom and my dad walk me down the aisle.
I would suggest that as an option so that way her father is included but you're also including her mom who is also a big part of her life.
I have seen a beautiful way to honor multiple family members by having the brides bouquet separated and her family members handing her flowers as she walks down the aisle. This way, she’s walking by herself but family members get to participate. She could also start with a single flower with a charm on it that has a picture of her grandfather in it. They sell beautiful charms like that on Etsy!