Nothing will help my DD in a tantrum so I just ignore her. When she seems like she is starting to calm down, I ask her if she want's a cuddle. If she's not ready, the tantrum spikes again. If she is ready she will come to me for a cuddle. She doesn't do it much when other people are around but if she does, I just explain to them that she will have a cry and get over it in her own time regardless of what we do or say.

DS was different. He responded well to cuddles as a way to reduce the length or severity of the tantrum. I would say to him "do you need a cuddle?" He would nod, have a cuddle then be fine.

Given your description I think she is more like my DD and despite their best intentions, your IL's are making things worse. The only thing you can do is explain it to them that way. Unless your DD is hurting herself, there's no harm in letting her have a little cry to get her frustrations out. It is also teaching her to get in control of her own emotions in a safe place.