DH and I are financially stable (poor compared to most DC standards, but stable income, etc) have great credit and low debt except for a very underwater mortgage. Our parents are all alive, in various stages of working/retirement, and quite comfortable financially. My parents support my sibling who is unable (aka unwilling) to support himself.
For a variety of reasons we would love to move (job proximity, schools, and space for DC#2). I think all grandparents are quite aware that DC#2's existence is entirely dependent on living space.
My parents have mentioned, once or twice in an offhand fashion, that they would be more than willing to "advance" us a portion of our "inheritance" if it would really benefit us more now... aka allowing us to move slightly closer to them, in a house which allowed them DGC#2 and a guest bedroom

They have not mentioned amounts. They have compared it to the considerable sums they have spent supporting my sib (again, without numbers).
DH and I have debated the advisability of pursuing this further. While I have no doubt that my parents could spare the amount of cash needed to make all that happen (really, 75-100k would be huge) we are worried about the family impacts. Has anyone BTDT?
-How the hell do I bring this up? How the hell do I say, "So, how much are you comfortable with?"
-How the hell do I say, "Please be aware that we are thankful for your generosity, but this does not buy you rights to input on our decisions"
-Did you regret it later? Were there family repercussions, ie really, really uncomfortable Thanksgiving dinners, etc?
Our relationship with my parents is cordial, but not terribly close. They are not in the immediate area, geographically speaking.
The other thread talks about the high costs of being old and sick... not to sound awful but what if my parents spend everything on my sib? Which is not inconceivable.