There is no problem attending - that is surely a filial duty that God expects of you - he is your dad!
I think this is a flawed argument, you are bound to obey your parents in
all that is not sin, her father is already married to his first legitimate wife, making this marriage no marriage at all but an adulterous relationship.
Sadly my older brother is a fallen away Catholic who is going to “marry” his protestant girlfriend outside of the Church, our priest has instructed us that we absolutely may not attend, as we would thereby be approving him in his sin, for his own good we have to refrain from going.
And I would like to share this quote from St. Matthew Chap 10:
"[35] For I came to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law.
Challoner notes: [35] I came to set a man at variance: Not that this was the end or design of the coming of our Saviour; but that his coming and his doctrine would have this effect, by reason of the obstinate resistance that many would make, and of their persecuting all such as should adhere to him.
[36] And a man’ s enemies shall be they of his own household. [37] He that loveth father or mother more than me, is not worthy of me; and he that loveth son or daughter more than me, is not worthy of me. [38] And he that taketh not up his cross, and followeth me, is not worthy of me. [39] He that findeth his life, shall lose it: and he that shall lose his life for me, shall find it. [40] He that receiveth you, receiveth me: and he that receiveth me, receiveth him that sent me. "