Leaving baby with SIL and boyfriend

My SIL just called and asked me if her and her boyfriend could take my 4 month old son for the day as she wants to spend some time with him before she leaves town. 
I asked her what she was doing and she said something. 
I am feeling really anxious about leaving him with her and more importantly her boyfriend who is a long term boyfriend, but I do not know. She or he would also be driving him around. 
I hate this. My husband has no problem with it. What do you guys think?