Why does my child mean nothing to my in laws

So I'm 22 +3. My sister in law is now 13+2. We both have one child to a previous relationships. Since shes announced about a month ago, my husband's family literally doesnt care about my son. I just the appt to discuss the results of our anatomy scan and he came back very healthy and measuring 2 weeks ahead and was almost a pound! I texted my mother in law thinking she would be happy and all she sent back was a thumb up emoji. Meanwhile shes posting all over social media about how excited she is to have another grandbaby in January and that she Hope's it's a girl. She doesnt even include the grandson that's due in OCTOBER. They were pissed hes a boy because "they have too many boys". Shes already bought items for my sister in law and they said they arent buying anything for our son until hes here. If they know anything about their grandson it's because I tell them, not because they ask but it's not the same for my sister in law. They posted her sonos but not my sons. I just feel like my son is gonna be second best to his cousin for literally no reason other than his fathers family are ridiculous. My sister in law doesnt take her kid to visit or to help our in laws out, countless times we go cut their grass, my husband fixes their plumbing, etc. I dont even want to invite them to the hospital or do anything with them because its obvious how they're going to treat him. Has anyone ever been through this?! How do I handle this constant feeling? It's so hard to smile in their face and pretend what they're doing is ok.

EDIT: SHES NOT THEIR DAUGHTER. WE BOTH MARRIED THEIR SONS

EDIT: He said something about 2 weeks ago but they basically said they love both kids the same. Which is obviously a lie. He doesnt really care for his parents so he doesnt really care. He is closer with my parents than his own and my parents are over the moon. They've already gone above and beyond. They call us just to ask about the baby. They bought his pack n play and outfits and stuffed animals and what not.