At 8 years of age he knows well this is wrong.
She said No, and he touched her anyway.
He used manipulative language with her.
I wouldn't want him ever near my child again, and OP, you will save yourself heartache by letting this relationship go.
This child is someone who doesn't accept No.
I would expect there will be issues of some sort going forward.
His behaviour is not normal.
It is off.
Unfortunately he has shown early signs that may well cause very serious problems in the future with pushy sexually inappropriate behaviour or sexual assault in his teens.
It always amazes me how people are so quick to play down and dismiss wrong behaviour as innocent when in fact it is a really early flag that something is very off.
I am so glad you are taking this seriously and protecting your daughter.
I would tell his father to treat this an early warning of possible disordered sexual behaviour and urges in this boy.
He didn't accept No, and he tried to manipulate her.
That is actions of a predatory child.
How many young girls are sexually abused by brothers who are barely in their early teens?
So many that you never hear about.
At nearly 60 I know of several cases where girls never told their parents, but left home and refused any futher contact with their sibling.
Others eventually told parents because they refused to visit and it caused unbelievable grief and upset in the family, tearing the family apart.
One dear friend of mine with 3 sisters that were abused, she wasn't, waited until both their wonderful parents had died before going to the police together and reporting their older only brother.
He was living a very respectable life, career with a family.
His wife and teenage children disowned him.
Thankfully he finally admitted it, sparing them a court case.
There is nothing innocent about his actions, absolutely nothing IMO.
Taking this very seriously and seeing if early intervention could help avert a disaster down the road.