If the mother is still alive, this is not an "inheritance" in the legal sense. It is a gift she is making (ujnless it is from a trust set up for the family's benefit; see below). Gifts are voluntary--if there is no trust or other legal document requiring disbursement to your husband, your husband's mother I(or whomever it is who is making the disbursement) can choose to exclude someone or delay paying them. Arbuably unfair, but legal.

If there is a trust or other legal structure set up governing the disbursements, then you need to reference the document(s) setting it up to see what is required and permissible.

If the mother is still alive, it might not technically be "inheritance," it might be gifts.  If that's the case, there's nothing you can do about it, because your husband has no legal right to get the same gifts as his siblings, during the mother's life.

It is possible that the money is going out as advancements against inheritance, if the family members are signing paperwork that acknowledges that.  In that case, what they get now will be subtracted from their shares of the estate after the mother's death, and the only difference between them and your husband is that they're making him wait for all of his share.  It's probably not something the courts would stop.