Left out of inheritance

My husband just found out that he was left out of his mom’s will. We moved his mother closer to us in an assisted living facility because his sister was moving to a different country. We had a fallout with his mother years ago and she didn’t want to get family therapy so our issues were never resolved. My MIL is now terminal. It was the right thing to do to move her closer to us since we’re the only family she has in the country, even though she’s a horrible person. My husband’s sister has known since 2017 that he was completely cut out of the will. Should we be mad at the sister who has known for years that my husband was no longer in the will but still moved the mom closer to us to take care of?

Edit: Everyone, thanks for the support. I think I need to clarify some things. My MIL was moved immediately to an assisted living facility in my town. She was moved across the country to be close to the only family she has left because my SIL was moving to another country on another continent. I pushed for moving my MIL closer in order to help my SIL feel good about their terminal mom being taken care of. My SIL is serving our country (not in the military). My MIL was truly awful. I witnessed her treating service people like garbage. EVERYONE is beneath her. You could google her name and read accounts of how terrible she was. Yes, she was mean but we don’t think anyone should die alone. Now she is just a bag of bones with a terminal illness and honestly because of the brain tumor, she’s actually being nice, isn’t that something? The betrayal is from my SIL not telling my husband that he was disowned in 2017. Let me make this clear. Evidently, my husband wasn’t “HER SON” when he asked his mom to participate in family therapy and she refused. He “wasn’t her son” when she disowned him and erased him from her Will. However, NOW he’s her son when he was asked to fly back to the original state where she was living because my SIL couldn’t handle their mother. My husband flew across the country three times to take care of his mom while running our business. We searched for the best assisted living place for TWO MONTHS to make sure everyone would be comfortable. My SIL knew this whole time that he was disowned but called on him constantly to fly out to help and also find the perfect assisted living facility. We were at the assisted living facility daily and my husband had to take his mom to the emergency room on three separate occasions. Since my SIL is the executor and has the power of attorney, we had to rely on her sending supplies like diapers, wipes, medicine. She would send supplies in small increments to our house so that we had to run things up daily. We asked her to coordinate everything with the assisted living facility but she didn’t trust them. We asked her to supply a hospital bed instead of the cheap wayfair teen bed that she bought, but she didn’t want to pay the $300 a month. To everyone who keeps saying “you aren’t entitled to your mother’s money.” You are correct. However, if someone decides to disown you, why do you have to be loving and attentive? I say you actually don’t owe them anything. My SIL knew this whole time that my husband was disowned but decided to plant their mother in our backyard to take care of. This is unacceptable and we would never have done that to her. One more thing, everyone is hung up on the money. It isn’t about the money, it’s just about being decent and honest. If you leave your child out of your Will, that is the final slap, the final F you. That says, “you meant nothing to me.” Then to have your sister be just fine with it and “oh, be sure you run those diapers up to mom.” Mom? “ Wait, I’m not mentioned in your Will, YOUR FINAL STATEMENT but evidently I’m your son when you need wipes and errands.”