G.T.
She's involved with a married man which means that all of her spare time is spent waiting on when she can see him.
It's not that she's ignoring you and your life it's that she's addicted to the weird life she has and it is restricting her without her even realizing it.
You know she loves you and your children but her lifestyle is such that she can't escape her tangled web.
I'd keep the door open for her as one day this will come to a crashing end most likely and you being the loving, caring sister you are will be there to help her pick up the pieces wont you?
My only sister is dead. Be patient. I'm sure she wishes her life wasnt so complicated and probably feels really guilty and bad for not being there for you, she just isnt at a point yet where she is ready to admit it.
jmo