Is my brother the father of her baby?

So my brother and his ex girlfriend broke up in August. She is pregnant and due either May 24th or May 21st( i cannot remember) He said they had sex the last day of her period which was August 23rd and again on the 24th unprotectedly. (pull out method🙄) Her lmp was either the 17th or the 20th but she’s measuring up to the dates of a lmp of the 17th & two days ahead. In her 4d ultrasound, the baby does look like him but I just don’t want to get our hopes up. I don’t have any reason to doubt her other than that they were broken up when she found out she was pregnant which was towards the end of September.