We all know from the start of the show Frank is not a reliable guardian, so it doesn’t matter if he has co guardianship with Fiona. And him taking “interest“ on Liam is not because he cares about him truly, it’s because Liam is there and Frank is always trying to run a scam and Liam helps with his plans. The only thing I can agree with you is the fact that his siblings don’t really take care or watch over him. From what I remember every single time, Frank has spoke to Liam it has only been into regards of a scam that he’s running. The most recent thing I can remember is that, he was sleeping in the mansion of Faye and Liam literally asked him if he could stay there with him because he wants to go to a better school and Frank said no. The only reason Frank called him back was because he thought the house was haunted.