This exact thing played out publicly somewhere.
The dad was powerful and had his money.
New lady, same age as daughter had even more money than dad- so she clearly went for his power.
The daughter of the dad, politely told dad that, dad, your fiancée is the same age as me.
Dad reassured the daughter that there was nothing for her and her siblings to worry about. But the whole public was unanimous in agreeing the daughter was right to point that out to dad.
Things are fine now OP, but you need your dad to be aware that should you need some advice from dad as your parent in future, anything from his wife, should be measured by dad with the consideration by him that you and GF are same age- jealousy, insecurity can rear their ugly heads if unchecked.
Q- what is your concern? Just the 'weird' factor or anything else? If you are concerned with inheritance, just speak to your dad.
Dad won't even engage in a full convo with you, but your discussion on inheritance and pointing out she is your age, will work as a guard when dad is making decisions- men can be mindless, without prior input and if not knowing your feelings.
Good luck. 50s is young! Like the 'handsome' dad too. My dad was very handsome too.