If it were every weekend that would be different, but every two or three months is surely not onerous? I guess it depends on your relationship. Did your sister ever invite you when her husband was alive? The dynamic has changed now. I agree that she will welcome a change of scene, to a home where she feels comfortable, secure and not lonely. Don’t deny those to her. It is not easy going home to an empty house. It may be too much for her to host you both.
But if you want to change the routine, you need to take the initiative. Suggests a day out or a short break together. Going out on your own, however confident you think you feel, is not always easy. The other day I was out in the next town, and felt very alone amidst families in the park, friends having a coffee on the pavement cafes, couples shopping together. Your sister values your company, but if having her at your house is too much trouble, make different arrangements to meet.