Obviously flying is safer for 3 out of 4 family members. The question is, is driving safer for that 1 family member. Can you compromise the safety of 3 family members for 1 family member, or compromise the safety of 1 family member for the other 3. Tough questions..there is no right answer.
I think this is just something where you have to go with your "gut". My instinct is that flying with the lap baby is safer (even for the baby) than driving. Driving is going to put everyone at a slight risk for the 30 hours of driving. Flying is only going to put the baby at a slight risk for a much shorter period of time. I also tend to think that car accidents are more likely to occur than "really, bad, turbulance..the kinds that would be dangerous to the baby"..but I don't know the statistics on that.
Does turbulance have anything to do with weather? Could you consult a farmer's almanac for that week and see if they are predicting "rain/wind or sunny/calm weather" the farmer's almanac isn't a great predictor, but I tend to think it is as good as anything else. LOL
Of course, you have several months...so you could always try your darndest to save up the $$ for an extra seat....get a credit card with frequent flyer miles and then charge all your groceries/gas/etc on it each month (and pay off in full each month). You might earn enough for an extra seat....or just try to sell stuff/earn extra money in the next few months.
Really...everything is a risk. Just going is a risk/benefit analysis. You obviously feel that the benefit outweights the risk, so you need to just accept that there is some risk involved in your decision to go, accept that risk, choose the method of transportation which fits your needs (monetary needs/length of travel time, etc) best and then move on.