The sister died 30 years ago, and we only have the siblings names not addresses.

I already have one sibling interested in a 12% return, no fees.

For all you know the person living in the house has 3 siblings located all over and during probate they may all own an interest in the property.In any instance this will take some time, there will be legal fees involved, and you will want to make absolute sure you are acquiring marketable title. 

What made it complicated is sibling are in a squabble over it and accused other siblings of taking advantage of someone whose competence is questioned.

Neither of my siblings went to college; both earn more than I do.

A non-lineal family member like a cousin or sibling could use their IRA to partner with you in some fashion.If the transaction will be a flip, it will be best for the IRA party to be a lender. 

My wife and her siblings discussed it, and currently her retirement income comfortably pays the taxes and utilities.

Good job in doing in finding driveway issue before closing.Personally I would like to get it resolved before closing, if the owners are siblings it will be easier for them to resolve it then it is for you, assuming they have a good relationship.Don’t assume after closing neighbor will do the same as w previous owner.

My two brothers and I will be inheriting close to 1m and and with them being younger (27 and 22) they want to invest this money since they don't have many uses for such a large sum of money now.  We are looking at inv...

I think there are more than 3 siblings involved.