In my experience, it is not normal for them to have taken all the guns in the house, even if your sister did not have a FOID card. Husbands pass away in this state all the time leaving wives and kids with guns and no FOID, and I've never heard of an immedieate siezure on those grounds. Even if they were taken to check for involvement in the suicide, they should have documented each gun and left a receipt with your sister.
Whatever the situation, please be respectful, but pro-active in establishing who took these guns, and where they currently are, and when you can expect their release to a family member that is a FOID holder.
Get the actual name of the officer/officers that took them, and make your sheriff/chief of police aware that they were taken.
The longer you let it linger in some administrative lingo, the more likely you may not get all of them back.
To the OP, sorry for your loss.
I have been in this very same situation. A good friend killed himself a few years ago in front of his wife. The local Illinois police dept took all his guns, including the one he used. The guns will all be held untill there is a coroners inquest, and after that has run its course, the guns will be released, but a friend or family member with an FOID will be required to pick them up.
In my case, I was able to take possesion of my friends guns. Eventually they released everything but the gun actually used, and then about two months later, they called his wife and released that gun as well. When I went to pick it up, I was quite surprised that the gun was still loaded with 5 rounds, and there was dried blood all over the gun. Does not say much for the protocols in place at the PD at the time.
I hope that his wife is more patient than my friends wife was. She did not want to wait to sell the guns via sales threads on gun forums, or through Gunbroker, and she took them all to a local Gunshop, and was basicly raped and was paid virtually nothing close to the actual value. Case in point, one of the guns sold, was a Browning High Power, and they gave her $200 for that one. I was sick after hearing about the details of the transaction.
She did not want the revolver that was used, and she told me to keep it. It creeped me out having it around, so I cleaned it up and sold it to an aquaintance for a fraction of its value, and I declared the circumstances of the guns background. As far as I know, he still has it.