Confused

Alright. Long story short my brother was seeing this girl who was in a long term relationship. So this girl and her boyfriend had sex the weekend of August 24. My brother and this girl had sex the night of the 26th. So she is now pregnant 🙃 is there anyway to do a dna test before the baby is born? She took a test and it was postive. Her boyfriend has no idea about her cheating or her being pregnant. If this is my brothers I want to be there for him even though he is NOT ready for a child. What she we do in this situation?!