I'm having problems with my husbands family, PLEASE HELP!!

Okay so my husband has 2 kids from his first wife. I have a child, then we have a baby girl together.Everyone in his family will take his kids out to eat, to the park, the movies, they will call me to ask when will we have his kids, or how are they doing, if they can stay with who ever it is, but they wont with the other two kids.Long story short his family treat's the 2 older kids like they are the best thing since ice cream and will treat the younger 2 like they r trash on the ground even tho the baby is his.They don't call to ask about then, to see if they can take them out, or even if they can come see the younger 2 kids.One of his family members even told me that i had no right to tell them what they could or could not do with the kids even if they were in trouble because i was not the mother and only there step mother.When it comes down to it she just came out and tole me i was good for nothing and was only here to watch the 2 older kids when my husband was at work or when one of his family members didn't want them other than that i'm useless. I have told my husband time and time again how i feel about this and he tell's his family that they can not have anything to do with the older 2 kids unless they have something to do with the younger 2 kids, but that makes me feel bad and it seems like.. well when a guy dates a girl just because he feels bad for her.It really upsets me that they would treat my kids like this even tho the baby is his. So they are treating his oldest daughter like she is a angel and his youngest like she is dirt, and just don't no how to handle it. If you can help or maybe think of anything please let me me.
Thank You