The death of the young mother is tragic, and - if you knew her - tears on your part are understandable. But a lot of your pain has nothing to do with her passing. You have been carrying a load of grief, which you are projecting onto this recent sad event.

Stop trying to be cheery and accept that you have been broken for sometime over something that was a great loss directly to you. If you can, help out in some way with the little girls who lost their mom. Maybe host a cookie decorating night at your house with your daughter and you giving these girls an evening where they get some substitute mothering. Don't do this alone. Do it with your daughter.

This loss is really more your daughter's than yours. She as aunt to these girls will want to step up in some way. You don't have to, but you might like to.

Sounds like some therapy might be very appropriate for you to start addressing your real underlying sorrow, which is not about these girls or their mother. You need care for that. You grief needs to be seen as important, whatever it really is about.