My 7 week old is constantly moving herself up the bassinet. We have her swaddled and a tight sheet over her the bottom of the bassinet, but she still keeps pushing herself up the bassinet. We have tried without a sheet but she stills moves up. Is it okay to let her sleep at the top or should we keep moving her back down?

It is not unusual for babies to wriggle & move around their sleep environment before they are rolling.
Some babies do not respond well to wrapping & may wriggle to attempt to get out of the wrap/swaddle.
Many babies like to touch their face or suck fingers when sleeping.
Red Nose recommends using a muslin or light cotton for a wrap.

She may be wriggling a lot because she is resisting the wrapping, particularly with the addition of the tight sheet to restrain her.
You could try wrapping her with her arms out - as with a baby around 3 months of age.
It is important that baby’s head & face remain uncovered & as you are doing, always place baby on the back for sleep, with feet at the foot of the bassinet.

If you wrap your baby, consider baby’s stage of development. Leave arms free once the startle reflex disappears around 3 months. Most babies eventually resist being wrapped. Wrapping style should be appropriate for the baby’s developmental stage.

It is essential to discontinue wrapping as soon as baby starts showing signs that they can begin to roll, usually between 4-6 months of age but sometimes younger. Babies must not be wrapped if sharing a sleep surface with another person. Baby should not be wrapped while sleeping in a baby sleeping bag.
https://rednose.org.au/article/is-it-safe-to-wrap-swaddle-my-baby
https://rednose.org.au/article/wrapping-babies
Depending on the size of the bassinet, she may be more settled in a cot.

Red Nose does not recommend the use of sleep positioners or anything that restrains a baby in a specific position.
https://rednose.com.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position