Seeking legal advice in Northern Ireland

Tl;dr - Chronically sick mum passed away and my Dad revealed he was having an affair. He now wants me to send him my Mum's inheritance so him and his new girlfriend can buy a house

My mum had several auto-immune diseases for 30 years causing her a lot of pain, muscle weakness and fatigue and constant medical issues. She was in and out of hospital regularly and in the last few years of her life, she wasn't able to walk or transfer out of bed herself.

As my Mum didn't want carers to come in the house, due to loss of dignity and worried that she would be mistreated by care staff since it happened before at home and in hospital, my Dad carried out all of my Mum's care included personal care, preparing meals, making sure Mum took her medication, organizing medical appointments and trying to stay on top of finances (carers allowance and disability benefits). My Dad told me he felt completely isolated and burnt out as he didn't have a life of his own and his mental health deteriorated a lot. I would come home from university every month or two to visit and help with Mums care, giving Dad some respite but it was nearly impossible during Covid-19 when Mum's health was at its worst due to the cancellation of most of her medical appointments.

Once restrictions were lifted I came back home to care for my Mum as often as I could but my Dad was in a pretty bad way mentally. Eventually we convinced Mum to let carers into the house as Dad couldn't do it alone anymore so she was given 4 calls per day and my Dad no longer had to make meals or carry out personal care and had freedom to leave the house when the carers were in. Dad said things were still difficult but he was feeling better because he started going for long walks to help his state of mind and increase exercise.

I visited my parents for Christmas in 2022 and although Mum's health was the worst it's ever been, she was in good spirits and had a good relationship with her carers. I thought everything was fine, until my Dad started acting strange. He would walk out of the house in the middle of a conversation, seemed really distant and would tear up a few times a day (I've only ever seen my Dad cry once before).

I was worried that he was hiding his depression and planning something stupid and said this to him. He said he was tired of this life and discussed moving my Mum into a care home (which we talked about before but I wasn't sure about since Mum never wanted that).

On Boxing Day night around 11pm my Dad said he was going for a walk, not that unusual since he walked after dinner most nights but it was very late and we were playing Christmas games. Half an hour later, my boyfriend came in to tell me that my Dad's car was gone. I called Dad, he didn't answer but then text me saying he was sorry and he just needed out. I was terrified that he was going to kill himself. I was calling him over and over begging him to come home and talk, he replied eventually saying he was going to their cottage for a few days because he just needed some time alone but that he was fine and wouldn't do anything stupid. I didnt sleep that night and around 3am my mum called for me to come into her bedroom asking where my Dad was (she was asleep when he left) and I had to tell her that he left for a few days.

I was supposed to fly back with my boyfriend the next day and visit his family for Christmas but I couldn't leave my Mum alone in the house with only carers coming in a few times per day. Dad didn't come back for a week and after that said he was staying in a B&B in the next town because he said living in the house with Mum was the cause of his depression. He never moved back home, but he did visit Mum 5x week for a couple hours a day. He told Mum repeatedly he would come home eventually just needed some time but he never did.

During this time, my Grandpa died and left my Mum an inheritance of £28,000. Dad asked if I could keep it in my bank account as it would affect their benefits, I was cautious but agreed because I had planned to keep it for Mum's care as I thought Dad would leave indefinitely.

A few other things happened before Mum passed away in September. I found out my Dad was living with another woman in the street behind my Mum and was in a relationship with her. He still lies to me about how long it had been going on before Mum died but he obviously went to hers on Boxing Day when I thought he'd committed suicide. He'd also brought his other woman into my Mum's house to clean, while she was alive since my Mum couldn't get out of bed. My mum never found out but asked my Dad every day if he would come home and he lied every time and said he would. He told me he was doing it to protect her and I would be left with the responsibility if he had told her the truth.

Fast forward to now, my Dad has been hounding me for mums inheritance so he can buy a new house in his new girlfriends name. I won't give him the money and he said I'm ruining his life by not letting him move on. He said legally the money belongs to him since he's the surviving spouse. This has ruined my relationship with my Dad and he said he won't put a headstone up for my Mum until I've given him the money. My siblings sympathise with him because he's had a difficult life but I don't believe he should use what was rightfully my Mum's so his new girlfriend can buy a house. He wants to take legal action against me now.

Does he have any rights to the inheritance? Would receiving the money make him liable for benefit fraud?

Edit: I should have mentioned, I split the money 3 ways between my brother, sister and I and allowed them to decide what to do with it. They both decided to send it to my Dad's girlfriend's account so that she could buy a house for her and my Dad. My Mum's funeral and wake costs were also covered with part of the inheritance. I have the remaining amount in my account which is what my Dad wants to take legal action against me for.