You are NOT being selfish, families, in their truest form, should really all be without all the step-fathers step-mothers step-sisters step-brothers..I know FIRSTHAND...Kaika I was soo hurt by my stepmother and stepsisters I have written here about it and they nearly killed mentally...if you have no problems with step-siblings on top of that well that's one good thing..step-this step-that!!
You sound sooo much mature then I was at your age...I love reading what you write, I admire you!
Perhaps when your around each individual parent WITHOUT THEIR SPOUSE you could talk to them in private about how you feel.....that you never had doubts of their love before this....that you count, too, in the midst of their blooming love lives they should not forget _Kaika!!
Remember you are so loved here!