My sister keeps smelling me what do?

Last time I lay on couch. Full length. She comes and there is no space for her so she climbs up and lays between me and the lean. At some point she asks if she can lay her head on my side. I have no problem with it and denying would be somehow weird.

So at first she lays with the side of her head on my side. Then she turns a bit so her nose is burried near my pit. When I notice I pretend to be adjusting my laying position and move until her head is like it was in the beginning. After some time it's already the same again. She tells me she's freezing and asks if she can use my arm to cover a bit (when I first noticed that my sister was doing so I did not touch her since then. I lay flat and loose and she has to move my arm by her own).

I am 18 and sis is 14 almost 15.

Basically, what sibling interaction is normal? Should I talk to her? or mom? what say? Pic is her.