Nervous

Just had my 3rd and last baby on the 29th and she is formula fed. My 4 year old has now started coughing and I'm so scared she is going to get the baby sick. 😩 my milk has come in and I'm trying to let it dry out because we wanted to formula feed but now I'm debating on trying to pump some milk to mix with her formula to maybe help her immune system. But I've never done that before with my other two kids and I also dont know how it will effect my breasts trying to dry out later.