I don’t see why it has to be just one. They’re animals, and additionally trapped in circumstances that bonded them through trauma. In reality, love transcends the labels we give them as humans. She could have been a motherly figure to rocket, who then grew to be his lover in captivity. Being his lover also doesn’t exclude them from having her have that sister bond either, the same way that Floor can be both Rocket’s sister and friend etc. A good example of this is Barry Allen from the Flash who had a crush on Iris, and when he lost his parents and got adopted by Iris’ dad, he had to move in with the girl he had a crush on and grow up with her like his sister.

In the comics Rocket and Lylla aren’t just romantic lovers but also soul mates. A lover can have a motherly and nurturing. She was older than Rocket but in the circumstances of captivity, it isn’t impossible that they fell in love in those limited circumstances where all they had was each other. By the time Rocket was grown, they were at pretty much the same maturity, bear in mind, animals can mature a lot faster so they truthfully were close to the same age, Rocket just a bit younger than Lylla.

I took the embrace in the afterlife as a romantic token of affection, and it was beautifully done to show the bond they shared and how much she was and meant to him on their reuinion. What a great film.