Taking of 2 kids is really challenging....tough in fact. It really depends on the age of the mom, if she is of high aged, probably doesn't want to wait too long. Otherwise, I personally prefer the big kid and the small baby has at least 2 years age apart. The advantages are:
1. The pregnant woman doesn't have to carry the big kid all the time as probably he/she knows how to walk.
2. The big kid probably knows how to wear his/her clothes, socks, shoes, go wi wi, eat food etc. Of course, it really depends on how much he/she has been trained to do so by him/herself.
3. To certain extent, the big kid could play games him/herself momentarily.
With these, the parents can pay a little less attention to the big kid, so to ease up the pressure on taking care of 2 kids who are of very close development milestones. This is especially true on weekend when likely your domestic helper is off, leaving the parents to look after the 2 kids themselves.
About jealousy, education is essential no matter how many years of ages the 2 kids are apart. Yes, if only 1 year aged apart, the big kid won't be able to show his feeling so much as for a 2 or 3 years kid...yet sooner he/she has the same kind of feeling that parents has to deal with.
To me, I'm very lucky, my big girl loves his little brother ver much about 90% of the time. And she knows how to prepare herself, like dressing up and preparing her backpack for going to school and going out with us. So what I observe is the more the big kid is train to do things independently, the less pressure is on the parents.