So my ex and I have a 3 year old daughter together and were together for 6years. At the end of 2018 he cheated on me and got the other woman pregnant and didnt tell me until October 2019 when the baby was already a few months old. We broke up and he got a dna test which confirmed that's his baby. I'm just wondering how and when do we (or just him?)explain to our daughter she has a sister? When should he introduce them? Is it better to tell her sooner rather than later? I never would have thought I'd ever be in this sort of situation and honestly have no idea what how to go about it. Has anyone any advice or experience with something like this? Thanks!

Edit-He is involved with the child but I'm not sure how much he sees her. He sees our daughter usually twice a week.