i love imaginative play, myself. but, what about dominoes/ matching games, puzzles? those are age appropriate and can be done at a table.
some ideas for you might be to do like other posters said and act out stuff from books - they make some animals that go with books that i've seen..
it's easy (esp since you're crafty) to make shadow puppets for that or just copied pictures from the books, too, maybe mounted on straws or popsicle sticks?
with book characters, you can go off- script, but some of the "hard" work is done- you are already acquainted with what the character is like, and the situation is sort of set.
also, what about something like a dollhouse? it might take some time, but if you have a collection of dolls / animals to go with it, i think if your kid can first develop the personality of the doll/animal, then the rest of the imaginative play might come more easily. and you can help elaborate on that. and the dollhouse can be at a short table for you to sit at comfortably! it just sounds to me like your kid needs help getting started, and hopefully once it gets established you can step away while the play continues...
i wouldn't worry about the sibling guilt. as a kid i was basically self-immersed in imaginary play b/c there was nobody for me to play with (my parents wouldn't) and i think i am much better for it- that's the thing with imaginary play: nobody else is needed. you can just make other people up if you need to!!! no limits to an imagination.
or music like pianojazzgirl said-- maybe act out the music somehow? one of my favorite things as a kid was the hansel and gretel opera- i loved to act it out... and peter and the wolf? or even the soundtrack to mary poppins or whatever...