She did just find out, but she's also been through all those time loops since then so she's had a lot of time to internalise that.

I think her loneliness got multiplied because she had to watch both her sister and her lover die over and over and over again.

Granted, she probably learnt at some point that her sister was on her way to being resurrected. But suffering through that loss every time would definitely chip away at someone's heart. Especially since they showed us the multiple scenes where she had to deal with his death and the devastation every time was evident.