House Mormont with Lyanna at his head are very firm in their support of House Stark and by extension Jon Snow.
Lyanna Mormont: House Mormont has kept faith with House Stark for 1,000 years. We will not break faith today.
Game of Thrones, Seaosn 6 Episode 7, "The Broken Man"
Also Lyanna is still young but she is incredibly smart for her age. As such I believe she recognises that the sword will do more good to Jon than it would to her. She's possibly too small to actually wield the sword effectively yet as well.
When the Old Bear gives Longclaw to Jon he also recounts that he's had the pommel changed from a bear to a wolf. As Jon hasn't unsheathed the sword in front of Lyanna it's possible she doesn't recognise it too.
Jeor Mormont: Good. You'll be ready for this, then. l thought a wolf was more appropriate for you than a bear so l had a new pommel made. lt's called Longclaw. Works as well for a wolf as a bear, l think.
Game of Thrones, Season 1 Episode 9, "Baelor"
The sword was also given to Jon from the Old Bear and as such House Mormont probably recognises it as a official change of hands, if they know about it. The sword was also meant for Jorah but when he brought dishonour on the house it was left without an owner.
Jeor Mormont: lt was my father's sword, his father before him. The Mormonts have carried it for five centuries. lt was meant for my son Jorah. He brought dishonor to our House, but he had the grace to leave the sword before he fled from Westeros.
Game of Thrones, Season 1 Episode 9, "Baelor"
Lastly, note that Lyanna is actually the niece of Jeor and is not of the main branch of the house, though through exile/death/Night's Watch her branch has become the leaders of House Mormont. As such the sword never would have normally passed to her but to Jorah.
On a side note in Season 7 Episode 6, "Beyond the Wall", Jon offers the sword to Jorah but he declines it stating that it is now Jon's sword after he forfeited his own claim. This also points out that maybe Jon didn't offer it Lyanna because he still believed Jorah to be it's rightful owner.
Jon Snow: Your father gave me this sword. He changed the pommel from a bear to a wolf, but it's still Long Claw. Lord Commander Mormont thought you'd never come back to Westeros. But you are back and it's been in your family for centuries. It's not right for me to have it.
Jorah Mormont: He gave it to you.
Jon Snow: I'm not his son.
Jorah Mormont: I brought shame to my house. I broke my father's heart. I forfeited the right to claim this sword. It's yours. May it serve you well and your children after you.
Game of Thrones, Season 7 Episode 6, "Beyond the Wall"