If it helps any (it probably doesn't) I felt very weird about dh's family holding my PFB. I didn't mind my family at all, but somehow couldn't see his family as being related to my baby. Even at the time, I was able to realise that this was an unhealthy, unfair and hurtful attitude, and I did allow ILs to cuddle ds1, but I didn't enjoy it at all. I rationalised it as a primeval protective instinct towrds my child; but let's face it, it was just baby hormones and a severe case of brain fudge madness. Reading your post makes me feel guilty for how I felt towards my ILs at the time, I like my SILs very much, and would like to hope they didn't pick up on how I felt (though they no doubt did), but I was very careful to make no moves to cuddle their children unless offered, just in case they were as freakishly odd as I am!
It's sad that you've been upset this way, and FWIW I think even I'd've leapt at the opportunity for a half hour's nap if my SIL's had offered to take ds1 for a stroll in the pram. But I'm afraid that at the same time, I can understand how she was feeling too. Still, I can reassure you that second time around is very different, and I probably practically threw ds2 at my family, SILs, BILs, postie, anyone who was passing really!