Show
Walder's eldest son Ser Stevron Frey is alive in the show and is now Lord of the Twins, Lord of Riverrun and Lord Paramount of Riverlands. In show there is no mention of Stevron's children but since he is a man of 40+, he must have Children and grand children. Lord Walder is said to have many unnamed Grandchildren, it makes sense that some of them might be Stevron's.
Since the info is not complete, it is not possible to draw a full line of succession. Among Lord Walder's Children we know only the following from Show:
- Stevron Frey, Lord of Riverlands
- Lothar Frey, deceased
- Black Walder Rivers, a bastard son, deceased
- Ryger Rivers, a bastard son
- Fat Walda Frey, deceased
- Roslin Frey, Alive
So by the known information, Roslin is indeed the first in line of succession. But that is only because we have no knowledge of children of Stevron, who would come before their aunt. We also do not know of other sons of Walder Frey, who would also come before Roslin in Line of Succession.
Books
In books Stevron Frey died of his wounds in Battle of Oxcross where he fought for Robb Stark.
As he was a grown man with Children and grandchildren, He was replaced as heir to Twins by his eldest son Ryman Frey. Ryman also has Children of his own, Black Walder who is portrayed as Lord Walder's son in show is infact a son of Ryman Frey and a great grandson of Lord Walder. Ryman also has two other sons named Edwyn and Petyr. Edwyn and Petyr have children of their own. Ryman was hanged by outlaws of Brotherhood Without banners and thus his first son Edwyn became heir to the Crossing.
The Line of Succession apparently would be:
- Ryman Frey (Dead)
- Edwyn Frey
- Walda Frey
- Black Walder Frey
- Petyr Frey (Dead)
- Perra Frey
- Aegon Frey (Dead)
- Walton Frey
- Steffon Frey
- Bryan Frey
- Fair Walda Frey
- Maegelle Frey
These are only the descendants of Stevron Frey. Other sons of Lord Walder have children of their own. A family tree of Stevron's line:
So according to Books, Edwyn Frey is now Heir to the Twins, he will be followed by his daughter Walda.
You can view other descendants of Walder Frey here. You can draw a line of succession yourself by keeping in mind that Children of an elder son come before a second son/daughter, a male comes before a female.
Roslin Frey's involvement
It is unlikely to see Roslin Frey who is not only a female, but the youngest female of Walder's children, to sit on seat of the Crossing. We already know Lord Walder has a number of sons and grandsons and men always come before women in their succession.
In Books Roslin Frey is loyal to Edmure Tully and while she did know about the Red Wedding, she and her full brothers had nothing to do with the Red Wedding. Her full brothers were not even allowed to be at the Wedding, fearing that they would let Starks know.
In Show, Roslin's involvement in Red Wedding is unknown. So is her loyalty to Edmure.
It'd be absurd to assume that Arya would ever do anything that strategical. She is not a politician or a general, just a traumatized child looking for revenge. In any case, Roslin won't be ascending to the seat because of her half brothers, full brothers and nephews. It is also unlikely that she would be able to influence her brother Stevron to abandon Lannisters and choose the North instead or to release Edmure. Stevron would never agree to it because:
- As long as Edmure Tully is alive and Free, Frey hold on Riverrun and paramount lordship of Riverlands would always be contested. Most of the Riverlander lords lost kin at Red Wedding and are loyal to Tullys. With death of Walder Frey, they might already be ready to rise up for Ser Edmure.
- Starks won't accept any offer of alliance for the twins because a huge number of them were involved in Red Wedding.
- Only Cersei Lannister can save the Freys from their enemies who would seek to punish them for Red Wedding. Cersei is their only choice.
Since Roslin's feelings for Edmure in the show are unclear, we cannot say whether she would be predisposed to release Edmure and forge an alliance with Starks in event of her ascent to the Twins. Remember, if she does that, she will be forfeiting Riverrun and Paramount Lordship of Riverlands to Edmure Tully. I don't think anyone would do that.
