Okay... ALL of my friends and my daughter's friends and my son's friends... have siblings.
When I have a play-date for my daughter, it is for her and her friend.
When I have a play-date for my son, it is for him and his friend.
Then, When or if, I invite a whole bunch of friends over, they and their siblings come. I state that. But it is up to ME. I am the Host.
At no time, do my friends, ASSUME, that the sibling is invited too or can tag along... IF it is for either of my kids.
And they make no drama about it.
It is understood.
What your friend needs to do, is to have or get friends for her younger son.
You, are NOT OBLIGATED, to invite your friend's son... to your play-dates for your daughter.
She is making you feel guilty and like the bad guy... and making you responsible for her life. ie: her husband works nights and her son is left out.
No. She is wanting you to be her BABYSITTER.
That is not right.
Do not let her, bully you into this.
Also, the age differences, is a lot.
I know... I have a daughter who is older, and a younger son.
And per your friend's son, she does not seem able to handle him.
She is, DUMPING all of her problems, onto you. And is using you as a way to dump her son onto someone else. Using guilt, and the play-dates you have for your daughter and her daughter.
Sure, you can be 'nice' and invite both her kids... but, she sounds to be really.... noxious. Gosh she left you SEVERAL rants... about yourself and your not 'babysitting' her son too and how it all bothers her.
She sounds... so full of drama.
NO YOU ARE NOT RUDE.
SHE IS.
She is off her rocker.
If that were me... I would NOT be friends, with her. At all.
Please, do not let her, manipulate you.
Unless you want her to dump her entire sink full of gripes, on your lap.