I wouldn't risk it. In case it really was just a courtesy invite and she fully expects you to not come it might well be very uncomfy if you do... for everyone. SIL wanting to tell you to leave but not wanting to since you drove 3 hours... and you guys feeling really lame because you took her at her word and came just to feel unwelcome.
I'd let DH go on his own and me and the kiddos would do our own thing. Maybe once theirs gets older and they get used to kids then maybe they'll be a little better... but why risk you guys being hurt, your kiddos being hurt... but then I am a majorly paranoid chicken who avoids confrontation... so *passes salt cellar*
Basically how my SIL is... I just do NOT feel welcome in her home, at all... I'll happily send something along, since FIL/MIL are going, but we'll be happy at home for sure vs possibly being incredibly uncomfy there... no contest. And the fact that with our risky cars we end up always riding with FIL/MIL and therefor cannot leave until they're ready... yeah, better to stay home... as I said, coward I am.