Hi again, Linda
My personal opinion on this is not from "breeding" experience - but from introducing a widely varied group to one another "one or two at a time"
Since I got two "sibling pairs" -- I kept the two together until I introduced them to the pride.
In ALL cases when I have brought a single kitten into the household, I ALWAYS kept them isolated for a period of time, JUST IN CASE the NEW kitty has a germ that the others may or may not have immunity to, or in case a "URI" is developing - which sometimes happens with re-locating stress, to avoid imposing it on established cats.
I would keep Lenny separate from Lubo and your "moggy" for at LEAST a few days - to a week, to make sure there are no "un-detected" germs, etc.
THEN I would re-introduce Lenny and Lubo, by allowing Lubo to join Lenny in his "isolation" room. As young as they both are, they will probably hiss a LITTLE, and possibly even puff up at one another, a few times, but by this time Lenny will be very ANXIOUS for a playmate, and it should go rather smoothly, over all. Let Lubo spend some "play" time with Lenny, and some time with your less accepting moggy, EVERY DAY for a few days, then bring Lenny out ONLY WHILE YOU CAN SUPERVISE until you are comfortable that he can hold his own with both Lubo and your moggy.
Having Lubo going back and forth between the two will ALSO help get the moggy used to the OTHER kitten smell, and allow Lenny to get accustomed to the other CAT smell.
Since your moggy is NOT quite as accepting - you might want to do things like give her a "favorite" treat, shortly after bringing Lenny into the main part of the house, WHILE he is "out" -- EACH time you bring him out.
Make much "ado" over her, and give her EXTRA attention, both before and after Lenny "visits". It will encourage her to look forward to the new family member "coming out". There is usually the "odd hiss or swipe" as they establish the pecking order. One will most likely be THE dominant cat, and the others will learn this cat's boundaries.
My guess, will be the younger two will wrestle and RUN and PLAY more, and then when they are EXHAUSTED from play, they will seek out the older cat, for snuggles, grooming and comfort.
I'm so excited for you !!
LaVon