I don't think it sounds like an ideal set up at the moment. You shouldn't feel pushed out of things like mealtimes and family games. It shouldn't be DH + kids vs you. He shouldn't be abandoning all other adult responsibilities while they are visiting. This is likely to create issues if/when you bring another child into the mix.
It is relevant to ask things like how mealtimes will work once the baby is there - babies don't sit in highchairs forever so eventually there will be five (or more) of you to fit around a table or fit into an eating situation, whatever that might be.
Although the other children don't live there all of the time you still need to consider them part of the family. Rather than being like planning for your first baby, where you can be quite haphazard and say ah well it'll be fine they'll squeeze in and maybe later we'll move - in reality you're discussing going from two children to three and they need to be catered for equally, you can't just squeeze the little one in in quite the same way.
I don't think stepmums should never have babies - far from it! But it is a totally different thing from planning your first baby as a couple without children and it's going to be 100 times more difficult if you go into it without being aware of that and what it means - and a first baby is ALWAYS difficult, even in the most perfectly fitting situation (which of course nobody has - everyone has some kind of complicating factor.)
Effectively at the moment you have a kind of set up where it's Dad + DC and then you - Dad's wife - separately where you're sort of remaining in the background. Then when DC go back to their mum it's you + DH again. If you bring a baby into this, how is that going to work? When SDC are there you'll still have Dad + DC and you - which one will the baby fit into? It's not overly likely that the baby will magically glue the two groups together as they aren't currently thinking of you in that way, which means it will either be Dad + DC (including new baby) with you feeling like a spare part and pushed out even from your own child which will be pretty upsetting. Or it will be Dad + DC where they go off and do stuff which is interesting to the older kids and then you + baby - and you'll probably continue to be stuck with all the housework etc - which will feel really lonely and crap. IME you need to get the SDC situation and not feeling like a family sorted before more DC come into the picture because otherwise the us vs them situation simply continues and that's really crap for everyone. You also end up with huge discrepancies around events like Christmas etc and you'll notice differences in the way DH parents his older two vs your child(ren) together because of the different amounts of time they have.
If you can get into a situation with SDC currently where they come and it's family life as usual (not putting on a special time for special guests), where Dad is just as involved as you are in household chores, where you are a part of their games and activities, where you get to know them and consider them part of your family, rather than simply part of his, then a new baby will likely tie things together, because it will feel like the four of you going into the new baby thing, rather than being you and (part-time) DH having a new baby which will feel very jarring because on the one hand you're expecting this first-time thing yet it's not really like that in reality.
No, I'm not suggesting everything will be plain sailing - but that's the biggest stumbling block you can overcome IME if you want to have a not completely horrible experience of blending families by having your first baby.