Please heed Annette's idea! She's right: Arrange some times and places just for the older boys to be alone together (with you). I like the idea of picking up JUST older boy from school and taking him somewhere with your son (and that will mean your 7 year old needs another place to be--can you plan some things for the older boys when 7 is at an activity of his own?). Or make the invitations for very specific things: "My son wants to go to the video arcade/play laser tag/the park with Your 10, and I'd like to swing by your house and pick up Your 10 and bring him over for that/take him to that with us." YOU would pick up and transport and it's clear the invitation is not for her younger son. If she expects her Five to get into the car and go with you -- that would be beyond the pale, frankly, and would require a much more direct talk with her!
For play at home, which your older son should not be denied because of the friend's sibling: "I know you usually bring Five as well, but tomorrow I have work that will keep me busy the whole time, and my Seven is at scouts/at soccer/doing homework and can't play during those hours. The older boys are going to be engrossed in video game/Legos/whatever, so tomorrow if you can drop off just Your 10 at 3 and pick him up at 5:30 that would be great." The clear indication being that Five is not part of the plan. If she turns up with Five in tow, greet him nicely, remain at the doorway as you usher in Older and your son whisks him away, and wave a polite and cheery, "OK, MomName and Five! See you guys at 5:30. I'd better get to work!" Seriously, doing the dropoff right in the door, or on the front stoop, telegraphs that you're going back in without them.
I disagree with the idea that you should send all four boys outside together to play. The older kids deserve time to build friendships without their siblings around and without being expected to watch or play with their siblings, period. Same for younger kids re: older siblings.
And oh, yeah -- you are being somewhat used for babysitting, either that, or she's a parent who just believes that her sons must do everything together. Either way, it's not working for you, so don't let it go on. She seems nice but possibly just socially clueless that kids do not have to accompany each other, and not all parents are open to having siblings along on play dates. (I'm not either but I've never had an issue like this!)