The eldest surviving Stark has been through the ringer during her GOT run, but is arguably the most equipped to raise a child. Having learned good habits early on from her mother and father with help from the devoted Septa Mordane, Sansa has also observed many examples of how not to parent: Cersei Lannister, Lysa Arryn, Roose Bolton, and the especially twisted paternal predations of the late Petyr "Littlefinger" Baelish. Sansa has proven herself an excellent leader who wisely learns from both the peaks and valleys of her own life experience, so her exposure to the whole gamut of Westerosi child-rearing styles will only serve to make her a better guardian for the Targaryen child.
Sansa raising Jon's child would also be a sweet form of redemption. She closely resembles her mother, Catelyn, who barely tolerated Jon during his upbringing at Winterfell. "All this horror that's come to my family, it's all because I couldn't love a motherless child," Catelyn laments in season three, recognising not only that her mistreatment of Jon deprived him of the childhood he deserved, but that her coldness toward him also affected his relationships with his legitimate siblings. Sansa adopting Jon's own motherless child and raising it with love and affection would be the perfect way to close out his fraught Stark family history.