It sounds like there are other heirs, based on the comment about wanting to be fair to everyone. If not, then yeah, Zamboni might want to decide how much of a fight--and strained relationships--this is worth to him.
But if there are other people, as executor, he's obligated to distribute things fairly and evenly (according to the will). So he can't just say, "sure, take the house, which is worth $150k, for your kid, and Asset, which is worth $75k, and everyone else can split the remaining $150k." It's not only morally wrong, it's likely legally wrong, too.
Thankfully, that gives an easy out. "the estate must be distributed evenly among the X parties named in the will [assuming that's what the will stipulates]. We will have the house, and Assest-SIL-Wants, and anything else of value, professionally appriased, which will be paid for out of the estate. Then I will add up the value of everything and divide by X, and that's what each person gets. If someone wants to keep a specific asset or the house, they can buy out the other parties by paying 2/3 of the value (or taking that amount less from everything else). If not, then we will have to sell everything. Once we have the values of house and asset, we will see if you are interested in buying everyone out, before I arrange to sell them."
Keep it business like. If SIL and Bro want to try to make plans to have some other beneficiary carry the loan on the house, they can arrange that privately. That's separate from everything else and shouldn't even be discussed in the same conversations. It should be, Okay, house is worth $125k and Thingy is worth $75k. If you want to buy out everyone for $50k each (or whatever it works out to based on number of parties), you can do so. Otherwise, I will arrange the sale."
If she then goes to sibling A and asks them to carry a $50k loan for her, and makes the same arrangement with sibling B and Zamboni, cool. Or if she gets A and B to let her owe them $50k, and lend her an additional $25k each to buy out Zamboni, great. Otherwise, it must be sold.