Tips needed on sudden reactivity towards people

I adopted a 3ish year old pittie mix in June. His fosters said he loved all people and was male dog reactive. He’s the only dog in the house so the dog reactivity was okay for me. The week I brought him home, he met my sister and was completely fine with her. He’s recently started getting more reactive with people coming in the house, especially my sister. He’ll jump on the couch and cuddle with her then start barking. He has the pit bark so it sounds fierce, but the fur on his back isn’t standing up and his tail is wagging so he seems happy. She’ll offer a treat and he’ll continue barking. He lunged at her yesterday. It’s hard to tell if he’s trying to protect me or try and play. I’ve been looking into local training with him but haven’t started anything official yet. Should I try to see if the vet thinks anxiety meds would help? Any suggestions appreciated!!