The "Other" Grandma

It's that time of year again. A few times a year Boy's biological father's mother takes Boy for a week of unabashed spoiling. I call her Boy's "Other" Grandma.
I need to say that I hate this. I hate this A LOT. I don't necessarily have a problem with the "Other" Grandma, I just object to the situation in general.
Basically (and as quickly as possible) Fuck-Wad (or FW as he'll be known from here on out) is Boy's biological father. He completely dropped out of Boy's life when Boy was around two. I haven't laid eyes on him in nearly ten years. He hasn't given us money in eight. The money that he gave us eight years ago was money he got by illegally claiming Boy on his taxes and came know where near the amount he owed us (or that he got from his taxes). But last August I let that all go because the courts ruled that FW had legally abandoned Boy and Mr. Hobbitfeet (my husband) was able to adopt him.
However, I have had this long standing agreement with FW's family that I would always let them have a relationship with Boy no matter what happened with FW. And I have honored it. Even though "Other" Grandma has talked at great length about what a wonderful dad FW is to his other son (though that is not quite what I hear from our mutual friends). Sometimes she'll talk at length about how they found FW's real dad but he's not sure if he wants to meet him because of his daddy issues, and they say this with no hint of irony.
I swallow my pride and move on because, quite frankly, Boy loves these people. They love him too, and frankly I don't see the harm in it. At least he gets to see what a creep FW is with his own two eyes, but from the safety spot of not actually having to spend any unsupervised time with him. That makes it better I think.
But still they come and take Boy away from me for week long stretches and I worry and worry, because he is (and always will be) my little guy (even if he NEVER let's me say that any more).