Ask at your local pint store there are finishes that are used for floors that keep the natural color but pine will yellow with time .

No stain and waterborne clear coat only.

clear coat, water base. Oil base will yellow over time.

Deft makes a nice clear coat that goes on well. it's water base.

You could use an oil like Penofin. it protects the wood and allows movement w/o cracking. (a Poly finish is harder than a varnish. if the wood expands and contracts a lot, then the poly could get hairline fissures. )

for my solid oak patio doors, I used the penofin on the interior. comes in a clear and a stain formula. (I happen to have used the ebony) It creates a nice seal to the wood, a nice finish and no cracking. There is no poly on this, just the Penofin.

this one has UV protectants. (if the window frame will get sunlight, go w/this)