If I was your SIL I would be quite offended. It's your nephews 1st birthday, surely you knew it was coming up and expecting an invite so 2 weeks seems quite sufficient.
My best friend didn't turn up to my DS first birthday - she text me with some lame excuse that morning. I was so upset and offended that she couldn't make the effort for my DS - he only has one first birthday!
If I were you I would pack a picnic and go. Not everyone likes everyone else's friends - but this isn't about you or your SIL or her friends. It's about your nephew and celebrating his birthday.