Babysitting at 35weeks

So idk what to do and still need to talk to my husband but my step sister just asked if I would baby sit for her. Her work schedule is literally all over the place. She works during the day, at night(over night). Her son and my son are only a few months apart and he's a good baby which isn't the issue. She's watched my son but after she gave birth( a few months). I want this time with my son before our new baby gets here. And with her schedule it wouldn't work with my husbands on some days. My son would love a playmate just idk if I want to do it. (We haven't discussed money and that's def not an issue)

Any opinions welcome