Uncle not Dad.. But they want to call me dad

Hey,I'm searching for some ideas,The short version of this story is..My wife and I were going to be the primary carers of my niece and nephew as my sister could not look after them as they should have been and we lost the ability to have our own kids because of cancer.Then suddenly my wife passed away with cancer and I'm on my own.My mum who has looked after these kids (now 5 & 8) for the past 3 years is also struggling terminal with cancer. She has turned these kids lives around 100% and they are truely great kids!In the past 3 years I have had ALOT to do with them and as mum is on her own I have been a fatherly figure and the male authority they needed coming from a troubled background with their mum.So the oldest (boy) randomly said one day "can u be my dad?" I was taken back at first because it was random and said that I can't be because I'm your uncle.Another couple of days passed and he just started calling me Dad.. I corrected him and said I'm his uncle.He then said to me that everyone else calls the person they look up to dad and I want you to be my dad.I really didn't know what to say/do but that conversation just passed in a second.I'm in a situation where they will be living with me permanently and have rare visits from their mum. But I don't feel comfortable with them calling me dad.How should I approach the conversation? It seems that every time one of their friends talks about their dad or Father's Day or dad days at school etc it comes up more and more often.Me personally after what happened to my wife and not having my own family is a sensitive topic that I'm still only dealing with, so when they call me dad it is uncomfortable...Any one else out there in a similar situation? And happy for any suggestions..Thanks in advance, via /r/Parenting http://ift.tt/2bv9DEl