Am I valid to feel disappointed in my stepchildrens mom’s behavior after she had another child?

    For context I am F26 and my Husband M25 have been together for 2.5 years and he has a son 6 and a daughter 4. My husband and their mom have a 50/50 custody agreement where we have them for one week and she has them the next. It’s Friday 5pm to the next Friday at 5pm

 In last week of December 2024, my stepchildren’s mother gave birth to another child. Since January 1st of this year she has had them a total of only 18 days. She had them January 1st to the 3rd. She also had them from January 10th to  January 16th where she asked us to take the kids a day early and we love having the kids here. We were supposed to return my stepchildren on January 24th. 

   Their mom had texted my husband refusing her court ordered parenting because the kids had a common cold, she said that she didn’t want them to infect her baby. We had them until February 7th which was at the end of our parenting time. She had them until February 14th. We returned them to her on February 21st. On February 25th she asked us to take them early cause she had surgery and so we took them to our house. 

  They were supposed to go back March 7 th but due to her baby contracting RSV where she blamed my stepchildren, they tested negative for it. In the end she refused to take them. Today they were supposed to go back to their mom’s house. We had let the kids know that they would go back to mom’s house today. From what their maternal grandmother informed me that her sister their mom’s aunt. Told her that when grandma took them to go visit great grandma in a nursing home that there was Covid going around. The kids were not exposed in any way to Covid. Their mom messaged and stated that she would not be taking them because we didn’t give the children multi vitamins and get them a Covid test. Their mom has given us so many medications for the children because she always says that they are sick. We don’t want to constantly pump them full of meds in order for them to see their mom. 

    I was in a car accident yesterday on my way to work, I do Medical Transport for elderly and disabled people. I was not seriously injured. I currently don’t have a vehicle to use to get the kids to school let alone to go get the Covid test. I had to call their grandma to take them to school and me to my office to fill out paperwork regarding the accident. I’m just so frustrated that the kids got all excited and then she makes us have to tell them that they can’t go. They love their mom an their baby sibling. They were extremely upset that they couldn’t go to their moms. They were crushed to the point where they were crying and kept asking why. It’s getting to the point to where how are we supposed to explain that we feel that their mom doesn’t want them around because she has another child.