Much to consider. But you already knew that.
Regarding upgrades for a higher selling price, I doubt that the will or trust suggested or required the siblings to launch a three-sibling joint-venture speculative real estate venture based on "Mom's property." Those are risky even with arms-length investors, but yours also involves family members and also in-laws chiming in with their opinions.
If you fix the place up and re-decorate, who decides just how far to go with rehab and expenditures, and who picks out flooring or makes decorator decisions? That is, of course, a minefield all its own.
Having one sibling "front" the $100k for rehab introduces an unequal aspect to the siblings. Suppose the one fronting the money wants special touches or finishes you believe are not appropriate to the local market? Since that person is putting up the money, are they presuming to be in control? Who manages the local contractors, which is a significant amount of time, and would (you) be reimbursed for that?
Instead of using money from one but not the others, if you instead chose to take out a loan, and hire a general contractor, does that even make sense given those added costs?
Later, when an offer comes in, does the one who fronted the money for rehab presume their decision about whether to accept carries more weight than the others? The slippery slope of "generosity" to put money up for development might have a long tail of downstream entitlement.
So this raises the question about what the will or trust spelled out. Those expressed wishes, depending on how detailed they are, could put some handcuffs on your decision(s) as executor. Or provide you with a shield of what not to do. Not sure how descriptive your Mom got?
Finally is the question of your authority as executor. Depending on what was written, that authority could have ranged from full, autocratic authority or maybe something lesser requiring consent or buy-in of others.
