I don't think you're venting, I think this is an honest to goodness problem that many of us parents struggle with too.
I think you need to talk with your family, maybe together so that no one feels picked on or singled out. Tell them that you'd really appreciate their help ensuring that the kids are treated equally. I know that my SIL has often done a wonderful job giving some extra attention to the "other" sibling during a birthday party. It doesn't have to be a gift or anything, but just some extra attention.
I'm assuming that your son has special needs based on your description of his medical issues and different school.
Maybe your son is simply going to NEED some extra help and attention since his situation isn't in the same school as his sister. You may need to explain this to your daughter...that you are trying to keep things fair and that you may have him help make a special Valentine's Day treat, because he isn't getting anything from his school.
We're running into something similar here at my house where the YOUNGER sibling is being invited to movie playdates, overnight stays, etc. The older sibling has been getting upset that her friends haven't been inviting her OUT more, just over for a playdate at their house.
I think we all struggle with life's little inequities. Just remember that as a parent it's our job to teach our kids that life isn't always EQUAL or FAIR, but it's more about how they handle it. Tell them that you will always do your best to treat them fairly, but sometimes, with schools, friends, et. they do have to learn to deal with disappointment or just say "Oh well". I know that's a very difficult lesson for a 7 yr old to understand tho.
So do your best to ask HIM what could he do that might make him feel better? Make a Vday treat with you? Invite one or two GREAT friends to a special birthday outing instead of his sister's big ol' birthday bash?
I try to explain to my kids that everyone is different. And that if you compare yourself to your sibling or friends you may be disappointed. Comparisons mean that someone seems to WIN and someone LOSES. Instead I'm trying to teach them to enjoy differences. Maybe if you focus on your son having one or two CLOSE friends, while your daughter may be popular, that can have it's own positives for each child?
I hope this helps.