If you liked your sister's 820, I would ask her if you could borrow it again. If you still like it more than the Marlin, look for a used 820 in your size or perhaps ask your sister if she'd sell or loan it to you until you can find a used one.

If you really want a new bike, the Trek Verve series has a more upright / comfort ride. However, I'd suggest finding the bike you know you like (820) vs buying a new and hoping. Also note that tires and pedals can be moved between bikes easily.

Edit: I looked at the Trek 820 and if you buy a new bike, I'd suggest one without a suspension fork, especially for your type of riding. A used 820 may still be a good idea if the fork is in good shape.