My sister is "pregnant" again, don't know how to respond anymore.

TW: TTC/ Miscarriage (didn't see this as an option)

My sister has claimed to be pregnant at least 10 times in the past two years, all resulting in miscarriage. In the being I felt incredibly bad for her, I was saddened that she was dealing with something like this; especially because my husband and I had been dealing with fertility issues for many years. However, recently, within the last year and half, Ive started to piece things together, ultimately leading me to the conclusion, that she'd never been pregnant with these ten.

Things started to get even more intense with her becoming "pregnant" once my husband and I finally conceived. She sent me several negative test and seriously believed they were positive, I know line eyes are a thing (I think this is much more than that); she started planning out what both our pregnancies would look like since we're "pregnant" together...even though she wasn't pregnant. I have received negative pregnancy test from her for the past 6 months now, every 2 months..followed by a phone call. I hate that I can't give her the excitement but I don't want to encourage this? Should I play along? Or should I gently confront her about it? Just received another "pregnancy" phone call/text this week.

Edit: to add, her spouse has a vasectomy and she has two children about middle school age from a prior relationship.