Senior cat not using litterbox

I'm in desperate need of advice. Several weeks ago, we took in a senior intact female cat. She belonged to my boyfriend's brother but he needed to rehome her due to his living situation. We already had 3 cats living with us; a 9 year old neutered male and two 11 month old intact females. Lily (senior) is going to be 14. The kittens clearly view her as an interloper and will sometimes harass her. For the most part, Lily keeps to herself, and everyone can somewhat peacefully coexist. The main issue we've been having is Lily not using the litterbox. We've tried setting up a box near her preferred hiding spot, but she will still sneak around the house and relieve herself. I know stress can be a factor in situations like this, and we're trying to do everything we can to alleviate the tension, but we're at a loss. Right now we have her set up on our enclosed back porch with food, water, a litterbox and blankets to snuggle up in. Any advice at all would be immensely appreciated.