Is AUTHOR wrong in the post ? POST : Firstly a bit of backstory:
 - my parents have been separated for several years now and we've all come to accept it. At first, I was terrified that I would never see my dad again (it wouldn't be the first time, more on that in a bit.) However, my parents defied the odds and remained friends, despite all of their hardships. 
 - Now, my dad is definately not the man my mum married nearly 20 years ago now, and that's partly due to his deteriorating mental and physical health. His head has dissapeared up his arse, to put it bluntly. 
 - When my parents split up, my dad moved out and he lives on his own in a flat that's not that far from us, like a 20 minute walk. Me and my 3 younger siblings stayed with our mum and he would come round pretty much every day to see us all.
 - Also, this needs to be mentioned for context. He's not my 'real' dad. He adopted me in 2003 shortly after he married my mum. For all intents and purposes, I'm going to refer to him as my 'dad' and my biological father as 'sperm donor' (because that's what he is, you'll find out why shortly.)
 - Another bit of context: recently, I got my first tattoo and it was a lot to do with my dad. I got my surname and the date that he adopted me, giving me that surname. He came with me when I got it done and noe its there forever.

Heres where my dilemma comes in. I love both my parents. I have a really close relationship with my mum and I used to feel like I could go to her with anything until recently. I also love my dad and I feel like our relationship strengthened when he provided me with much needed support when my sperm donor fucked off. From when I was about 11, until I was 16, I had a relationship with him, but then one day he suddenly up and deserted me, giving me little other reason than he didn't agree with some of my life choices (I have my head shaved on one side, I was bisexual and I had a girlfriend at the time.)
My dad helped me a lot when this happened and I was so glad he was there for me, considering my parents had split up a few years prior.

Recently, I feel like my relationship with my mum has been going downhill, and it may be partly to do with the fact that her and my dad arent on speaking terms at the moment. This has happened a few times but every time, its only lasted like a week, and then they start talking again. They haven't spoken since Christmas. Christmas day was the last time he was round at our house. My mum is on a shorter fuse lately, and i feel like that's because of his deteriorating health. Shes said to me several times that she thinks he's being selfish because he's not doing anything about his health for the sake of his kids. He'll be on deaths door soon if he doesnt do something and she thinks he should try for his kids. But he'd terrified. He's been told he has 18 months to live (I'm not going to go into details as to why.) that was a hard blow to all of us, him most of all. He doesn't know what to do with this news so the best thing he figures to do is not think about it. But that's not gonna make it go away. 

I feel like I've been drifting away from my mum and gravitating more towards my dad, even though I can completely see both sides of it. But every time my mum rants about what a useless prick my dad is, it makes me wonder if getting my tattoo was the right decision. Rather than being angry, I want to try and make the most of the time I have left with my dad.

TL;DR: my parents have been split up for years now and my dads health is deteriorating. He was given 18 months to live which made my mum angry because she thinks he should try and do something about it for his kids. I got a tattoo to commemorate my dad adopting me but I'm starting to feel weird about it because my mum has fallen out with him. I also feel like I'm drifting away from my mum and towards my dad, but I feel like this is wrong, given the circumstances.

So, am I an asshole for choosing sides between my parents?