6 months is both a growth spurt and a huge developmental leap. For both my kids, their sleep was all over the place when either/both of those happened.
The only thing that worked (and only worked with one of my kids
) was to get in there just before the 45-minute mark and try to get her back to sleep before she woke fully. In our case she was spitting out a pacifier at 45 minute and that woke her, so it was a matter of re-plugging before she noticed it was out. After a few days of that, she extended on her own. I don't know if something like that would work for you with 2 other kids but might be worth a try.
In the end with my DD2, though, I ended up wearing her for a lot of naps just so I could have a life. She slept better on me, and DD1 needed time with me too, so some days we just worked it like that as best we could.