What advice should I give my husband?

This is a long post and it is about my husband and his father. And before you go saying it is not my business, I am asking because my husband asked my opinion and I really don't know how to answer him. A bit ( a lot) of background is needed, but I will try to keep it brief. My husbands parents divorced when he was 9, his sister is 3 years younger than him. Their mom had them for 2 years living with random guys and ignoring them. Their dad finally took them away after she was leaving them alone for days and not feeding them. I know the divorce happened because she pushed his buttons and he had a temper. Growing up he would hit and choke my husband. His sister never experienced physical abuse. Growing up, my husbands sister became a replacement wife imo. His father dotes on her. She is a very angry, spolied person, prone to outbursts at everyone-even her father. During these outbursts, her father tries to calm her calling her darling and catering to whatever has made her angry. My husband has always been ignored or pushed aside. My husband and his sister grew up and moved away from IL-she to CA and my husband to WA. His dad and sister see each other monthly either her flying to him or he to her. She calls him multiple times a day. As an outsider, I just think it is a weirdly close relationship. I don't suspect anything sexual, just emotionally close. His dad probably sees us twice a year. My husband and I have 2 children and I know my husband is disappointed that his father chooses to visit his sister so often and rarely sees our children. His sister has a dog and a cat which she has elevated to the importance of children within the family. She calls them "his grand kids", constantly posts pictures of them, etc. and if you point out to her that they are just pets, she gets very angry. Whenever she goes on vacation, she normally flys their mother out to watch the animals. His father just retired last month and as usual was out visiting his sister last month. He is going to be visiting us for Halloween which is awesome, and his sister for Thanksgiving. His sister is going on vacation for two weeks at Christmas so his dad is/was also going to be spending Christmas with us. However, his sister asked his dad to pet sit for the two weeks she is going to be gone. So now his dad will be spending Christmas and New Years at her house, watching her pets and then coming back to our house after New Years. My husband is very upset and views it as his father choosing to cater to his sister instead of spending time with his grand children. I know part of it is because his father is afraid of his sister and wont say no to her. My husband wants to tell his dad that if he goes to not come back. I can understand, but I also think that nothing is going to change their weird relationship and that giving an ultimatum like that isnt going to change anything. So my question is, what advice should I give my husband? What would you say?