My Mom + Dad are both 95 now. Dad's sister-in-law has often liked to complain about things. It seems like Dad is getting sucked into her complaining, maybe and giving her money. I ask why he's giving her this money and he says, "Because I can!"What can I say / do to get him to hopefully stop this??
I say that if your dad is of sound mind and if it's not hurting your parents any financially, then just let him give his money to whom ever he wants to. Because he is right when he says..."Because I can!"....and he rightfully can.
Are you POA for finances?
If you answered NO to these questions then your dad is free to do with his money as he wants.
Now if you want to start a fight that will tear the family apart you can try to get his doctor to say that he is incompetent and then you can go to a lawyer and seek Guardianship. However since your mother in the picture she would also probably have to be declared incompetent and you would have to get Guardianship of her as well.
Now to be serious....
If there is any remote possibility that either your mom or your dad would need Medicaid then the gifting needs to stop.
If you are worried about your inheritance and the fact that your sister is getting money and you are going to be left short changed when mom and dad die wait until they die and you can find out if it is all equalized in their Will.
Keep in mind this is THEIR money to do what they want with it. They earned it, they saved it they can spend it.