I keep thinking that instead of adding this old uncle, they should have added a younger maternal uncle (brother of their mom and aunt), that way MC and his sister could catch another brother/sister in the act and develop their relationship in a very natural way.
But there's no need, I don't understand why someone would even want to see someone else have sex with one of the LI in the comic. To do what you say, there are 2 option or more.
1) The first option is The sister see her brother with they aunt doing sex/love.
2) He could have ensured that the same passion and taboo were consumed in another house (neighborhood). The MC and his sister could see a brother and sister do sex through a window and then start looking at each other differently and little by little get closer in the same way.
There are many ways to add spice to the story. If you want to add new people and therefore other couples. The important things is not to add NTR in a story where is NO NTR till now.
There is no need to bring other men into the house where the MC lives. It's all very simple and it worked great with Satisfying Needs. Where the aunt see her nephew fuck his mother (her sister).
Why ppl wanna always NTR in every story? And again, Deval can always created a new comics. One not related to the three story he already start, something new just for NTR fans/sharing. But ruin one of the old comics is just stupid.
Does it take a long time? Create a new story. And already on the first story you create a cover with NTR written on it, like they do in Japanese comics. You have to say it from day one and not after the story has already started three chapters.