Step parents/bio parents

Through my entire life my step mother always treated me like her own child, until recently

I have a three year old son and a 4 month old son, she was ecstatic to be apart of our lives and was my toddlers greatest joy in life, 2 days after i had my second son we discovered that my sister was 22 weeks pregnant (her daughter) and the past few months have been a struggle because my step mother literally walked out of our lives over night, my three year old crying all the time because he wants his grandma to come visit. And i feel like im trying to force a relationship with her she no longet wants.

Today currently my sister is in labour, i told my son aunties in the hospital having baby and grandma is there helping her, he cried and cried thinking auntie was sick (we recently explained death to him when my husbands grandmother passed) i asked her if she could give us a call as soon as she got a free moment to help ease my son of the fact auntie was fine. I got a text msg back "ummm your sister is in labour, and you fucking know it, tell him to be a big boy and go to bed" my hear was crushed for him 😢 i know from this point on things need to change im just not sure which way to go. Hes still young enough to forget if i put some major effort but she still is my mom and i keep holding on to the one shred of idea that it might change back to normal yet

Just looking for some opinions on the situation, feel free to ask questions, im up for answering anything, it might just give me something to ponder on over the next few days while i decide