First thing is to separate them into their own cages immediately, right now if you haven't already. If you don't they are both going to get severely injured or killed, as will their chick. If they haven't shown any aggression towards the chick yet, it will happen, and it could be from either of them if you leave them together...Mom can raise/feed the baby without dad as long as she has ample food, water, and a Mineral Block/Cuttlebone to keep her Calcium up.
This happens with birds, and sometimes there is a distinct reason and sometimes there is no reason at all, at least one that we can see or that makes sense to us...But that's just it, we as people tend to kind of humanize our pets, and we assume that a "breeding pair" of birds that are bonded are going to be like a pair of people that are a couple with a child, and that's just not how this works. Budgies are not monogamous with one mate when other mates are available, with male Budgies in flocks going from one female to the next to the next. So it could be that the chick caused some jealousy, it could be that Igor is wanting to hurt or kill the chick and Shiva isn't letting him, it could be that Igor has become excessive in his preening of her, and now he's wanting to over-preen her or even start to mutilate her, it could be hormonal aggression, or it could be something entirely different. Sometimes we just don't know, and all we can do is keep them safe and protect them from each other.
You may or may not be able to put them back together into the same cage, but definitely not until the baby is fully weaned and moved out into it's own cage...And it will need to be moved into it's own cage after it weans, because if it's a male it may start breeding with mom, and if it's a female it may start breeding with dad, and you don't want any in-breeding...
My suggestion is to keep them in separate cages sitting next to each other so they can see and talk to each other. When the chick fully weans it needs to go into a totally separate cage from the parents, and I would start out gradually by only letting Igor and Shiva be together outside of their cages, and the same goes for the baby if you're going to keep it. And if you see any aggression between the 3 of them outside of the cage then you'll need to separate them and not let them out together ever unsupervised...You may not ever be able to put them in the same cage again, sometimes it's just not worth the risk...
It's hard, I know, but they just aren't people...We had a newer member rescue a Budgie from death inside of a dumpster, he brought her home and did an amazing job bringing her back to life, and she bonded with his current male Budgie, and they had a clutch of babies. He was very clear that he wanted them to "live together as a family" and "raise their children as a family", and then one day he came home to find the female had decapitated one of her chicks. This absolutely crushed this member to the point that he really hasn't been back on the forum since it happened about 2 months ago. We tried to explain to him that parrots are not humans and they don't have human relationships, and that what happened is completely normal, etc. But the last thing I saw him write was that he was re-homing them all, he just couldn't take having to separate them from each other or keeping the chicks separate from the parents or each other, even though we told him they were all going to start in-breeding if he didn't separate the parents from the chicks, and the chicks from each other. So his solution was to re-home them, because he wanted them to have a human-like family with the same type of dynamics and relationships, and that's just not how birds work. So your only job is to keep them all as safe as you can, and right now that means keeping Igor and Shiva completely separate from each other in their own cages and only allowing them to be together out of their cages under supervision, and if they show aggression towards each other out of their cages then you'll need to not allow them to be out together at all...Same with the baby, as you don't want more chicks that are in-bred. This could be temporary and things may go back to normal once the chick is weaned and in it's own cage, but it also might not.
