I am sorry for your loss. Although not the same, I had to explain the loss of our kitty to my then two year old this past year, and he is also pretty bright for his age. I found the following advice, which applied to the loss of a pet or a loved one:
The advice I found was to be honest, but simple in explanations. No "Uncle is sleeping" or "Uncle has passed away." It is hard for adults sometimes to say the person died, and a child this age doesn't really have a concept of death, and doesn't understand a euphemism. Or maybe your child does have a concept of death, maybe he knows about bugs dying,for instance, that they don't move anymore after dying, and you can relate it to that -- once your body stops working, you have died, remember what happened when you stepped on that ant? So, the explanation should be geared to saying, uncle died, his body stopped working. Depending on how uncle died, and if he wants to know how he got broken, you could explain that -- uncle had a very bad sickness that some people get, and it caused his body to stop working, the doctors could not help him, that sometimes happens -- remember when we could not fix that toy you broke? Uncle was in an accident that caused his body to break. You might have to repeat the same thing over and over, as he asks. Don't provide more information than he requests, keep it literal, keep it simple.
Then again, if he was not close to the uncle, one explanation may be enough. Although my child is bright as well, they still don't "get" death at that age. He did not get upset when our kitty died, and kept talking about her for a while as if she was still around, but he now gets it. I had to keep telling him that Mynx died, that we won't see her anymore, that we miss her, but her body broke. He did notice that my husband was very very sad, so he would say "is daddy sad about Mynx?" and I would say yes, and then I would ask if he wanted to hug daddy to help make him feel better or something. That was the extent of it.
Just be careful to be straight, and be aware that explanations involving angels and heaven may be confusing, because of how literal children are. If you tell him uncle is sleeping, he may expect uncle to wake up, and/or become afraid that either he or you won't wake up after falling asleep. I might be wary of explaining that uncle died due to being sick, too, in case that might disturb him the next time he gets sick. If you say uncle "went to heaven", he may take that as a place that uncle can come back from or that he can go visit. Your call on whether your guy can handle a discussion involving heaven, if you believe in those things.
Primarly, he may begin to connect that if uncle's body can stop working, mommy or daddy's body might stop working. Kids at this age very much need reassurance, they are very self centered, and their main concern at that point would be who will take care of them? So tell them who will take care of them, or if neither you or your husband are at death's bed, it was suggested in my research that you tell them that nothing will happen to you or daddy. You don't have the sickness uncle got, you are very careful drivers (if uncle died in a car accident) etc. Tell him that aunt blah or whoever will be happy to take care of them if mommy or daddy could not. What they want is to be reassured that someone will continue to look after them.
I would be most concerned with what he will see at the funeral home, because that is more immediate. Prepare him for and tell him that he may see you and other people crying. If there is an open casket, tell him that everyone is looking at uncle's broken body to say goodbye, that this is one way that people say goodbye to people who have broken. The crying of people around them sometimes upsets kids more than the person's death, and tell him that you are crying because you are sad that you won't see uncle anymore, and that you are going to miss him. That it is ok to cry when you are sad. Then, if you want, and he asks a question that naturally leads that way, you can talk about your religious beliefs.
I think this was all very sound advice, and my little guy handled it pretty well. I didn't need most of it, we didn't have to spend much time on it. However, that was a cat, not a person. They do keep asking, periodically, so be prepared to answer the same questions many times.
I doubt that the 1 1/2 year old is really going to understand much of anything. He may just be upset at people crying, and I would just hold him and comfort him as I would at any other time that he would get upset.
Many people offered advice about explanations involving heaven, and I think that they did well with their kids, I particularly liked the story of the balloons taking Valentines to Pap Pap, but again, caution -- if they can go there, why can't the child? Again, cognitively, kids are very literal. How can there be a place that you can't get to? So just be prepared for that, if you choose to include a metaphysical explanation (religious) in your discussions. That might be harder to comprehend. If he asks or hears things about heaven, then for sure, I'd talk about what you believe with him.
I hope that this helps. Funerals are such a mixed experience -- you are sad, but at the same time, they are joyful -- you learn things about the person that you never knew, you see and connect with people that you haven't seen for a long time. I like to think of them as a goodbye, and a celebration of the person's life. That's the Irish way of looking at it -- that is why they cry, drink and dance at a wake.