Congratulations on going no-contact Dec 3rd thru 5th. Three days, I believe that's your longest streak! Unlike your ultimatums, this is the first sign you're beginning to detach and her backup plan will slip away soon.
I hope you heard what you needed, "It's not all good over there." I'm sure the truth lies between the flowery account she told you and the venting account she told her sister.
Regarding the sister-in-law, I would encourage shutting down conversations like this in the future as they will generally hurt your detachment. You heard what you needed to hear. Now, let go. This doesn't mean you can't talk to your sister-in-law. This just means to shut down any conversations concerning your ex.
May'19 - separation. | Dec'19, Oct'20, Jan'21 - painful breakups. | Jan'21 - freedom! "We the ones who play hard, we live hard, we love hard, we light up the dark." -- Kesha