If she feels she can't take a weekend away from her kids, that doesn't mean that she doesn't attach as much importance to the relationship as you do. She might appreciate everything you do, but she might think that leaving her kids (especially since they are young) is too risky. If you could arrange a babysitter or relative who could be trusted with those kids... then perhaps that's your ticket I don't think you're being selfish... but more that you're just seeing it from a different perspective. You said you love your own boys a lot, but you'd also have to be sure of their own well-being before going off for a weekend, wouldn't you?

Or if she really doesn't think she can leave her kids until they are older, then why not think of an alternative way of treating her? Perhaps it could be a day trip out somewhere, taking the kids with you? Or maybe arranging a babysitter for a couple of hours and taking her out for a meal?