Digestive28 is spot on re hormones & meds. I have OCD, agoraphobia, several other more specific phobias, depression & anxiety generally.
I tracked my periods for a different reason earlier this year (they were starting to become less regular) and discovered my anxiety MUCH worse premenstrually and depression was worse post menstrually. As a result with cpn support gp has agreed I can take pill continuously and just not have the damn things!
However, from my own research & discussion I'm also firmly of the belief that many are being Dx with depression who are actually suffering from a PHYSICAL illness - hypothyroidism, nutrient deficiencies etc so your sister might want to get these checked if she has any symptoms that fit. It might even be the case that's not the whole story but isn't helping.
My Cmht have started doing blood tests on recently referred patients and longer term patients who have possible other symptoms and they've found a significant number aren't truly MI but have other conditions where depression/anxiety are symptoms.
Other than that as per pps everyone is different.
While seemingly loving to "pop round" etc - I hate that! And so do many with MI. Unexpected visitors (even loved ones) can be additional stress.
So I would say listen & respect what SHE says about how she finds it best to cope.
Mind have some good resources too for friends/relatives.
You could also educate yourself on any meds she's on if you know - but remember that's her private medical info so don't ask but if she tells you great.
Honestly the best people who supported me were/are the ones who GENUINELY don't mind if they get a phone call/text at 4am saying help! I'm having a REALLY bad night. I don't do it often and haven't for years actually but I am SO grateful to those people they literally kept me alive!
Meds can take a good while to kick in so if she seems despondent in early days of taking remind her of this and that taking them regularly (at same time/s every day) is essential. BUT if after a reasonable time they aren't working or making things worse support her to get them changed - which can be REALLY difficult!
The basics seem insignificant can REALLY help - eating well, sleeping routine, exercise if possible, sunlight, fresh air.
Depressed people often lack motivation eg for cooking, self care but maintaining those where possible or finding easy to manage alternatives can really help.
Eg I'm really struggling with cooking/eating at the moment so I'm making sure I get in pre-prepped fruit n veg to try and keep nutrition from getting COMPLETELY disastrous, also fruit juice (meds can also dip my blood sugar and this is quickest way of bringing them up again too), and "fresh" soups.
Focus on positives - I do this self talk thing (doesn't always work) where I remind myself that while I feel I Cba brushing teeth/having a wash/brushing hair/changing clothes when I've done it I always feel so much better!
You are lovely for caring enough to seek more info on this. 