Hi D.,
I would invite your mother-in-law over for a morning coffee break or something, when your husband and kids are not home, and then gently broach the subject with her.
Don't get mad and act resentful, that will just make it worse.
Simply explain to her that, since you moved to their town to be with your husband and his family, that they are now a part of your own family, and since your blood relatives live too far away to do things on a regular basis with your children, you would appreciate it if she and her husband could step in and help out.
Nicely explain that your kids feel very left out, and that you would like them to learn that, no matter if they are blood-related or not, they are still important in your husband's life, and are now part of his family. Also, your stepson should learn to accept them as his family, and as long as they are being excluded by your husband's parents, they will never truly feel a part of the family there, which can be very hurtful and bad for their self-esteem. They may even become resentful of their stepbrother because of this.
It could very well be that your in-laws aren't sure that you would even want them to spend more time with your children...if you don't tell them, they won't know. They might think you wouldn't want them to be 'replacements' for your own parents. Tell your mother-in-law that you don't want replacements, you just want your kids to be included and feel like a part of their family, which they are now.
Hope it helps...good luck and Happy Holidays.