In the Tv show Sansa is married to the Bolton bastard son (Ramsay) who gets legitimised by a decree of the King. Ramsay then kills his father and "becomes" lord of Dreadfort. That same "bastard" (Ramsay) then gets killed. He has no heirs so is Sansa now the Lady of Dreadfort?
This is simply unknown as of now.
Lord Ramsay Bolton left no living children or siblings. His Father Roose Bolton is not known to have any immediate kins either. Therefore it is unclear whether a Bolton claimant exists. Even if a Bolton claimant exists, why should they be granted Dreadfort? House Bolton is guilty of treason against the North and House Stark. Starks have every right to revoke their titles as their liege lords. But then again, only the senior line of House Bolton is guilty of treason and no longer exists in any case. It is possible for the Starks to be merciful and diminish their role from powerful vassals to small Lords or Northern equivalent of Landed Knights just like Robert did with House Connington where he demoted them to Landed knights but granted the family seat to younger line of the House. But it must be noted here that while the Senior line of House Connington fought for Targaryens, the younger line redeemed themselves by fighting for the Rebels in Robert's Rebellion. That is not the case with the younger Bolton lines, if any exist.
If we take example of the books, that Shall be decided by the Overlord of Dreadfort, that who becomes the new Lord or Lady based on different claims. That Person is Jon. Precedent here would be the Succession Crisis of House Hornwood (Never happened in the show), it was to be decided by Robb Stark.
But since this is about Show heresy, anything is possible.
If we look at Show-precedent:
1. Olenna Tyrell becomes de-facto Lady of High Garden due to her marriage to Lord Luthor Tyrell and death of all her Children and Grand Children, therefore wiping out senior line of House Tyrell. Olenna Tyrell (nee Redwyne) then became de-facto Lady of the Reach.
2. Cersei Lannister claimed the throne once her Baratheon Children had died even though her only relation to Royals was based on her marriage to Robert.
So if they can use marital ties to claim a title, So can Sansa.
Yes Sansa will become the Lady of Dreadfort because she is a widow of Ramsay Bolton and neither Ramsay Bolton nor Roose Bolton had any heirs. Since this event has not happen in the books, I cant find quotes to substantiate the arguments.
There are at least couple of instances of a widow taking over from their dead husbands. One such instance if of Lady Donella Hornwood.
Lady Donella Hornwood:
Lady Donella Hornwood brought no tail of knights and retainers; only herself, and six tired men-at-arms with a moosehead badge on their dusty orange livery. “We are very sorry for all you have suffered, my lady,” Bran said when she came before him to speak her words of greetings. Lord Hornwood had been killed in the battle on the Green Fork, their only son cut down in the Whispering Wood. “Winterfell will remember.”
A Clash of Kings, Bran
But who will take over Dreadfort, after Sansa will be complicated. In the case of Lady Donella Hornwood there were complications:
“Maester Luwin answered. “With no direct heir, there are sure to be many claimants contending for the Hornwood lands. The Tallharts, Flints, and Karstarks all have ties to House Hornwood through the female line, and the Glovers are fostering Lord Harys’s bastard at Deepwood Motte. The Dreadfort has no claim that I know, but the lands adjoin, and Roose Bolton is not one to overlook such a chance.”
A Clash of Kings, Bran
Roose Bolton forcefully marries Lady Hornwood and claims her land.
“The old knight was off east, trying to set to rights the trouble there. Roose Bolton’s bastard had started it by seizing Lady Hornwood as she returned from the harvest feast, marrying her that very night even though he was young enough to be her son. Then Lord Manderly had taken her castle. To protect the Hornwood holdings from the Boltons, he had written, but Ser Rodrik had been almost as angry with him as with the bastard.”
A Clash of Kings, Bran
Taking this from a bit more of a practical perspective than the other answers; hereditary claims are usually taken by those that claim them (depending on circumstance to a degree). For example, Cersi's claim to the throne in a patriarchy is tenuous but in the face of unclear alternative leadership, she takes the lead and benefits from it.
On the other hand Sansa is off toddling about Winterfell playing Jon Snow's cheerleader. I imagine that no one at the Dreadfort would give a hoot about Sansa's technical claim and are probably (after holding a recovery morning after the massive piss-up from breaking open the Dreadfort's cellar now Ramsay is gone) squabbling over who in the currently assembled court will take the seat. Once that's done then it becomes another de facto taking of the seat, similar to Cersi's.