All the FIL and BIL of Pandavas fought in the war for them.
For Yudhishtir we have a Sivi king fighting and dying in the hands of Drona in ghatothkach Vadha Parva. (Sivis were also having same problems as Kouravas and pandavas. One side which was ruling the territory at the time fought with Kouravas. The other / cousins whose daughter had married Yudhi fought for Pandavas)
Similarly for Bhima we had Kasi king father / brother of his wife Valandhara fighting for them. But their cousins the Kosalas were fighting for Duryodhan who had taken a bride from there.
We also have Ghatothkach and his son fighting for Bhima
For Nakul we had chedi army led by his brother in law dhristaketu and his son fighting (Karenumati's brother)
For Sahadev we had One half of Magadha fighting for him led by his brother in law Sahadev. But his brother Jayatsena with other half of Magadha army fought for kouravas.
His other wife Madra princess Vijaya's father Dyutimat and his son srenimat also fought and died for Pandavas but his brother Shalya went to Kouravas.
For Arjun we had his wife Ulupi's tribe led by Iravan.
For Manipur also a lot of scholars believe that this was actually Manalur, capital of Pandya kingdom which is locationaly more accurate as nothing south orissa where Vaitarni river and Parshuram's mahendragiri are borders. So this Pandya king fought and died for Arjun. In any case in southern texts he is father in law of Arjun and his daughter is chitra.
Subhadra had only Krishna coming in and her son Abhimanyu. You can count satyaki here her cousin.
So all the Sasural's came but it is surprising that sons are not mentioned at all.
One reason that is applicable for Ghato's other two sons are they were too young.
If all these boys were born around Rajasuya then they must be 14 or 15 and hence considered too young to fight.
But Abhi was 16 or 17. Though at that age 1 or 2 years also make a big difference
Or these guys were the actual 5 uppandaavs who died in the war