Before anything else, get an answer on what the son legally is. An heir in possession of estate property and a tenant at will are removed by different processes, and in some states an heir with an interest in the property can't be evicted through the ordinary landlord path at all until the estate distributes or the court orders possession. That distinction is state law and it belongs to a probate attorney there, not to your spreadsheet. But it decides whether your worst case is four months and 3k or eighteen months and a partition fight.
Price the delay rather than pretending it away. Six months of vacancy on that unit plus legal is realistically 8k to 12k, which is five or six points of your 215k. If you hold your 155k you're absorbing it. Conditioning on vacant possession is the clean answer and it often kills the deal, because your PR is the occupant's sister and won't be the one to force him out. That family dynamic is the actual obstacle here.
The middle path most people land on is a seller-funded relocation payment negotiated pre-close, commonly 3k to 5k, signed by the occupant with a firm move date, with a matching escrow holdback of 15k to 20k that releases when possession is delivered. Make sure all three heirs consent in writing, since a PR who acts against a beneficiary's interest invites a challenge that lands on your title.
On unit A, in many states deposit liability follows the property whether or not funds were ever transferred. Take an affidavit from the PR stating no deposit was collected, take a credit anyway, and expect no verifiable rent history for financing. Also expect the appraiser to be refused interior access to the son's unit, which shows up as a limited inspection and can restrict your lender before the occupancy problem ever does.
