It wouldn't be that she "doesn't value a long friendship". It would be that her DC's come first. You WILL understand if and when you have DC's.
My DD will be 16 this year. My DS1 will be 12. My DS2 will turn 11 at the end of the year, and my DS3 will be 3 later this week. Each if their Birthdays is just as important to me AND THEM as the previous one.
I have never worked on their Birthdays, I've ALWAYS booked the time off, up to a year in advance, to ensure that I am with them on their Birthdays.
The only wedding that would take priority over one of their Birthdays would be...one of my other DC's wedding. And I would be very unhappy if one of my other DC's chose to get married on one of their sibling's Birthdays.
I will still do my best to spend time with my DC's on their Birthdays (their lives permitting) even when they are 56! So it's very presumptuous of you to think that she should delay one of her DC's Birthdays for your Wedding.
Why can't you book your Wedding for the week BEFORE her sister's wedding, to ensure that she can attend both Weddings AND be there for her DC's Birthday?
Would that not be a better solution all round, if you haven't fully set the date yet?
It's almost like you are trying to 'test' your friendship with this woman by attempting to make her prioritise you over her DC. That is just unfair.
If she chooses not to attend in favour of being there for her DC's Birthday, it is not because she doesn't value your friendship. However, she may well value the friendship LESS if you get shirty with her for prioritising her DC's Birthday over your Wedding, if you book them to clash.
If you made that comment to her about not 'valuing the friendship' by choosing not to attend, I'm quite sure that it WILL damage the friendship.
Why would you try to make her choose between you and her DC like this when you could just book for a different week, like the week before her sister's wedding, and show HER how much you value the friendship and want her there?