I currently watch my almost 10 month old nephew mostly 4, sometimes 5 days a week. While I LOVE my nephew and like the idea of him and my son getting to play together, I never really wanted to watch him full time. Especially since the original deal was supposed to be 3 days a week. I felt trapped into agreeing since I am family and couldn't see how feelings would not be hurt if I said no. His parents are "different" than my husband and I, they never have a set schedule, bring him anywhere from 7:30 a.m. to 10:30, and he stays anywhere from 3:00 till 6:30. I never know the plan, it drives me insane. I'm pretty passive and just resorted to texting them every night to ask when/if he will be coming.
Anyway, I have already told them I will not be able to watch him when the new baby gets here, they were understandably disappointed, but the thought of a 2 year old, a 16 month old and a newborn was stressing me out. I chickened out and told them I would let them know how I was feeling and see how I could handle watching him later on in my pregnancy, when I really wanted to stop watching him in February when he turned 1.
Does that seem too unreasonable? Should I wait until April or May? I really want some time to enjoy this pregnancy and to hang out with my son before he has to have a sibling full time. I know I need to be more specific with them so they can find alternative daycare, but I wanted to see what you gals though was a reasonable time for me to quit.
Thanks:-)

Re: Thoughts on when to quit babysitting.
I think @mullenem gave great advice. Keep in mind that they'll have to find another situation so notice is key.
@mullenem Thanks for that well thought out response. Yeah, it's my BIL and his wife. I know he thinks he's doing me a favor bringing him late some days, and it IS my fault for not saying anything. And yes, they are paying me, not always on time, but eventually. That part doesn't stress me out though, they are having a hard time financially, and I don't let that part bother me.
I really will give them a specific date, like you said, I just have to figure out when I'll be the most comfortable with it.
@uganamafans matter of fact is so hard for me! But you're right, I will be definite.
@mullenem