How do you deal with your in-laws favoriting their other grandchildren over my children?

I have 4 children ages 4-14. My husbands sister has 2 sons. My husbands parents have shown my kids pretty much no attention since my sister in laws kids came along. It really truly bothers us. My mother in law buys constantly for those boys, but NEVER buys anything for my kids. She sees them every single day, and sees mine about once a month, even though we live 5 miles away! We went over their on thanksgiving, and they pretty much disregarded my kids, and payed most of their attention to the other grandkids, even though they already see them everyday! My husband has talked to them on several occasions on how it bothers us, and how wrong it is to not spend any time with our children, but nothing changes. Just 3 weeks ago my husband called them because my 4 year old was saying that he missed them and wanted to go to their house, and told them maybe they should do something with him that weekend, and they never even called back!! It makes me sick. We just don't know what to do.