Having Sansa in between them also offers the sword-wielding Starks a chance to brandish their signature weapons, putting Sansa in a prime position to be protected.
It is also possible Game of Thrones wanted Sansa in between Jon and Arya to not ruin the presumable Season 8 reunion between the siblings/cousins.
Seeing Jon and Arya next to each other could have been seen as diluting the build-up to the characters’ much-anticipated reconnection.
Emilia Clarke's Unsettling Reaction To Daenerys' Final Game of Thrones Scene