In season 4 episode 7 Fiona makes bail and goes home. She had been arrested after Liam overdosed on cocaine left on a table at Fiona's birthday party.
Fiona's lawyer wants her to take a plea deal which might mean 3 months in jail and 3 years probation. Talking it over with Lip Fiona keeps insisting that she is innocent. She is charged with cocaine possession (she admitted it was hers and she had snorted some of it at her party) and child endangerment.

Of what does she believe she is not guilty? Possession? She is guilty of possession. She did not buy it or seek it, but when it was left at her house she didn't throw it away. Rather she used it and shared it with her friends. Child endangerment? Fiona defends herself saying she had only turned away for a moment. But guilt is not determined by the lack or presence of intentional neglect. Fiona loves Liam and never intended any harm to come to him. Yet because she kept the cocaine in the house instead of throwing it out, because she left it lying on a table unattended, and because she was not watching Liam to prevent him trying the cocaine himself, Fiona endangered the child. She is guilty on both counts.

The sad fact is that Fiona is the most responsible adult in the family and does more to care for and protect those kids than anyone else.

I am disturbed by her insistence that she is not guilty because she didn't leave the coke out for Liam to consume on purpose. Lip correctly replied to Fiona "Yeah but you did it". Fiona "I turned my back for like...2 seconds, and now my life is ruined, how is that fair?" Again Lip counters with the reality that Fiona sought to become and was named as Liam's guardian, and with that position comes the responsibility to prevent such preventable tragedies.

The situation is tragic in many respects. Fiona is best suited among those available to be the guardian of those kids. Yet she failed to keep an illicit controlled substance away from her three year old child. She now faces 5 years in prison unless she takes a plea bargain, and winds up with a criminal conviction. If she has simply flushed the cocaine that her boss's creepy brother left in her house instead of keeping it this nightmare would have been avoided.

And it is tragic that she refuses to acknowledge that she is guilty under the law.

It isn't surprising though. The show is billed as a tragicomedy about a dysfunctional family. At this point in the show it is more tragedy than comedy though.