There can be a variety of reasons why people don't 'do' funerals, and if they're getting on in years it could be that your auntie and your uncle can't face confronting their own mortality, or their respective dh/dw's have a problem.
However, as they wrote to your dad it really isn't your place to take them to task or get involved in any way unless your pa specifically asks you to.
It's a 
fact that births, deaths, and marriages can cause deep rifts in even formerly close family relationships, and if you exacerbate the situation it's possible that you will alienate these particular relatives to a point where they won't wish to attend any future family events or keep in touch.
How does your dad feel about their non-attendance? If he's not particularly close to his brother and his aunt it's unlikely he'll derive much comfort or support from them, and he may not be too upset or surprised by their non-attendance.
Your immediate concern should be supporting your dad through this very difficult time, but if you must meddle do it by phone calls rather than letter, and lay a massive guilt trip on them.
Keep your voice sweet and reasonable with a slightly sad tone (practise in advance), and do not give in to any temptation to sound sarcastic, become angry or say how you really feel something harsh, Say that your dad is obviously devasted by his loss, and was so looking forward to having their arms around him at this difficult time as were you.
Conjure up some happy memories of them and your mum; say how much she admired/respected them, enjoyed their company, often said...(stretch the truth), and how much they'll be missed next Tuesday. Ask them to think of your dear mum and their brother/nephew hat the relevant time.
In other words, keep faith with your mum and your dad by taking the moral high ground and let these twats come to terms with their disrespect to your mum and disloyalty to your dad. .
Personally, I'd be as angry as you at their lack of respect but it would give me more satisfaction to make them squirm whenever I communicated with them, and keep taking their gifts, than to cut all ties with them - I'd quietly look foward to attending their funerals or sending my apologies if it coincided with a holiday not too many away/interfered with my sewing club/hairwashing/pub quiz etc.
I am so very sorry for your loss; please go easy on yourself and don't feel that you have to settle scores at this particular moment in time; what goes around comes around, and all you have to do is wait, watch the wheel turn, and give it a little nudge now and again.
Wth deepest sympathy to your dad, and to yourself and your family.