I have 3 little ones w/ my DH. He has two teenage daughters, ages 14 & 16. We are in a much different situation because I don't have to take on a mother role with his children at all. You are & do because they live with you full-time. I think any woman who takes on that role deserves a standing ovation. You are concerned about your skids already so I know you're a good Mom to even consider the challenges for the family.
The best I can do is give you some advice as to what I would do if in your situation. I don't know your skids ages but I would involve them as much as possible. I would tell them you are thinking about giving them another sibling & get their take on it. Mention what good siblings they are. Ask them about names, take them to the ultrasounds, have them pick out some clothes, get them as involved as Dad. If the baby kicks have them feel your tummy first...especially hiccups. Make it totally fun for them. I would be open with them about the jealousy issue. I would tell them sometimes that they might feel that way but reassure them you love them dearly & that it's normal. I think in all families their can be jealousy. Caleb, one of the twins(22 months), was so terribly jealous of Nathan when he was born. Caleb wouldn't even look at me at first. I tried to include him as much as possible but he wasn't having anything to do with our new little intruder. Now Caleb is protective of the baby(9 months) & totally loves him. I don't think your skids will treat a baby as an outsider for too long if at all as long as they feel connected with the baby too. Actually, if they are old enough, they'll probably feel sorry for you having to change diapers & meet every need of the baby. I think you should totally go for it. It'a a beautiful thing.
P.S. Are the skids boys or girls or both? If they are boys, it might help to let them pick out some new toys like balls, hammer rattles, or blocks. I think little girls have a natural draw torward babies. Little girls(stores, restaurants) are very googley over my boys when they see them.

I have often wondered the same thing newstepmommy. I have 3 skids and 1 biokid from before I met my dh. My skids bm also has more children with her new man(I think you said something about their bm not being around but having other kids). They are jealous of those kids because BM has those kids all the time but ignores them. And when my son's dad has missed visitation days with him and my son complained they have made comments like "well at least you get to see both your parents all the time".

So I am nervous too that they would resent a child that me and dh had because it will have both of us full-time and they don't have that. I mean they have us both full-time, but not their other bioparent.

But one plus, is you won't have bm poisioning their minds against siblings. (I have heard many stories of an ex telling children "those are not your siblings" when their other parent remarries and has kids) At least they will all grow up together as siblings. I would think that would make it easier for them to adapt to eachother. And I really liked Misit's ideas on including them and getting them excited about it all.

DD has one brother (her dad's and 2nd wife) and the other one is on the way to be born this summer. No, there was no jealousy. DD is very happy to have siblings. don't ignore your other kids when you have babies and it should work out OK.

7 years ago my DH got emergency custody (now has full) of my SD a month before our DS was born. I thought this would be a huge jealousy/insecurity issue, and BM tried very hard to make it one, but for some reason we came out unscathed. We had another DS 3 years later and still no issues, other than things like why do I have to do dishes and the 3 and 6 year old don't . . . normal family stuff.

SD sees her mom every other weekend, and mom just had a baby 15 months ago. There is major jealousy there, but I believe it's because of the way BM and stepdad handle it. Her stepdad dots on his daughter and ignores his SD. SD is forced to baby sit and care for the baby often so mom and stepdad can watch TV together, take 'naps' together, etc. She resents that, obviously. There is a definite difference in that home between the girls caused by the way the adults (new g-parents included) treat them.

So, I think it's all about the way you handle it. My kids are all my kids, and they are all treated the same. No mention is ever made to 'our kids' and 'your kid' or anything of the like. In our home I discipline, comfort, encourage and 'ride' each kid the same, and SD (now 13) has several times commented that now that she sees what life would be like if I treated her as her step dad does, and she appreciates me. I think even her mom is seeing the difference a step parents attitude can make. Fine is right - if you treat your skids the same as you always have, and treat your baby as your treat your skids you should be fine.