how can i support my sister when i think she's made the wrong discision?

[supportivesister]

am a regular but have name changed for this
this is long one so bear with me
my sister lives a 2 hrs drive away from the rest of our family
she has recently given birth to her dd 6 weeks ago ,she has a ds aged 5 too ,2 weeks after the baby was born her dh dumps her ,she is devastated and has been diagnosed with pnd for which she is having ad's for .he left the family home and she just cant cope ,she wasnt having the baby at nights as he was looking after her at his mums after my sister said she couldnt cope at night times although she still had the baby during the day ,
her dh is turning nasty and threatened to call the police on her if she didnt stop harrassing him,she was hysterical and has decided to leave her kids and come home to live with our parents
my question is how can i support her when i think she has done the wrong thing by leaving the kids ?

spent the day with my sister on monday and she seemed well
laughing, joking ,playing with my ds
eating well
very strange
she didnt cry at all but did say she missed her ds no mention of her dd
her ex has brought him a wii -so that makes it all better ????
he has been very naughty at school and they are getting him social worker or someone like that
i can see him getting suspended for his behaviour.poor boy he must be so upset
my sister hasnt so far been to the doctor,have suggested it and so has our other sister ,the eldest sister cant speak to her shes too upset by whats happened and says she will shake her to get her to see sense if she saw her so she has decided to stay out of it.
seeing her again tomorrow .
im hoping she will realise what shes missing

i have an update
my sister is feeling so much better and is now desparate to have her kids with her
is spending the whole half term with her son and is determined to have the baby too ,she is looking for a place to rent ,the local council wont give her anything,they say that she is still housed in their eyes as she still has a mortgage.
im so relived