Okay, where you have community property and kids outside of the marriage that existed on the date of death, 1/2 goes to the surviving spouse and half goes to the kids of the decedent. So her daughter should own 50% and his five kids own the other half in undivided interests. That is also how the property tax debt should be split: 1/2 to her daughter and 1/5 each to his five kids.
The deceased sibling's share will pass to that person's heirs. It will be separate property though, if they were even married, since it was inherited. The property would go to the deceased siblings children equally, if any, with the surviving spouse, if any, having a life estate in 1/3.
If the daughter got a deed after probate then her interest is secure. The affidavits of heirship should work for the others, including the deceased sibling's heirs, provided the title company and the lender involved in the sale will accept them. An affidavit of heirship is the least reliable form of conveyance of decedent's real property, so sometimes lenders and title companies won't accept them. But it's really going to be up to them. I would avoid the estate administration route unless you are required to in order to sell the land. Even then it might take too long to get an administrator appointed before the county forecloses on their tax lien. If that was going to happen then I would pony up the cash to pay the taxes and get reimbursed later when the property sells. I would avoid the county's foreclosure sale at all costs.
To answer your question, though, in the event you do have to get an estate administration (you would have to have two in this case - one for the stepfather's father and one for his deceased child's interest), then you do have to get consents from all of the heirs for an independent administration. Costs of administration are born by the estate, so any money in the estate, including proceeds from the sale of real property, would be used to cover the costs, which will be mostly the attorney's fees. Some attorneys will require a retainer up front, so someone might have to come out of pocket for that. I typically would not in a probate or estate administration because the estate is going to have money enough to cover my fees and I didn't want to make an executor or administrator come out of pocket for that before they got access to estate funds.