Could it be his baby ?

My sister just found out she is pregnant. She had sex on the 16th and 29th of December. On the 20th of December she got her “period” but it only lasted 2-3 days and didn’t feel normal since her period is normally longer. She passed it off as a period. Well she had sex with someone else on the 3rd of January and is hoping it isn’t his baby as it was just a one night stand with a guy she went in a date with. We just found out today that she is pregnant. Is it likely she got pregnant on the 16th? We’re making an appointment but there aren’t any appointments available until the 7th so thought I’d ask here. She’s 30 and just got out of a 10 year relationship and went a little wild and we’re just trying to figure this out

Just an update! The doctor confirmed that she did conceive around the 16th of December and the bleeding was most likely related to pregnancy even tho it seemed like a period. So it is definitely the first man’s baby.