You’re welcome for the insight, I’m glad that you find it useful!  It’s very understandable that you could start to worry about the future and how her current sadness or grieving might impact her mental health going forth.  That can be a slippery slope though, so as much as you can, I would offer that you try to focus on the current situation and remind yourself of the factual information you have about her rather than to start thinking too far in the future and whether or not birthdays or holidays will be effected.  Even if she does have to spend half holidays with you and half with the other sister, remind yourself that she is an adult capable of making these decisions and for managing the feelings that come with those decisions– you cannot feel things for her.  I sense that your intention is in the right place though and that you’re very worried about her.

I would suggest asking her directly how you can help her get through this process.  If you feel comfortable doing so, I would let her know how much you care about her, state that you are there for her and want to help, and reiterate that she has your support through this process even if it is hard.

Ask her specifically what, if anything, you can do to help her and then truly listen to what she does or doesn’t need.  Seeing people we love suffer can be very hard to sit with and we often just want to make it better for them as soon as possible.  But how you can help her best is to fully understand what she wants and needs as well as what she does not want and need.

She can speak best to her needs around her mental health and perhaps she’d like your help with certain things and not others (maybe would like help finding a therapist or taking her to appointments or maybe she just wants to have a weekly visit or call with you…options are really open here and a lot can be done)

It will likely help her so much to just know that she is not alone in this and that she has your support and to know that you can sit with the uncomfortable feelings of sadness and grief with her without trying to necessarily fix them for her.  Feelings come up and will dissipate eventually once they are able to be fully felt and accepted.

I hope this helps!!