I hope that some of the links Sophie_M gave you have been a little help. I'm sorry for the loss of your cousin and more sorry again she took her life.
Reading your story I imagine you have had some experience with the grief you feel when someone passes away, particularly if they are elderly or had been unwell. There is loss and a part of you that is no longer there. It's natural, and there is pain, and it has no schedule or timescale.
At least with those feelings you can share with others, you are both feeling much the same and it is expected, a natural part of life.
Suicide is very different for those have been touched by the death. On top of the normal grief is whole new layer of unexpected and horrible thoughts for which one has no experience and nothing to help cope with them.
The facts are that your guilt is misplaced. Saying this to you will not make the feelings go away at once, but as you mull it over in your mind you will find there is nothing you missed, no sign that could be taken, and no actions you could have taken. Your cousin had her reasons, right or wrong , accurate or otherwise, and sadly those reasons went with her.
Talking to those close, they will be in a same position or worse, with stronger feelings of guilt, perhaps feelings of betrayal or abandonment, even anger on occasions. All perfectly natural, all perfectly as it should be. They too will be looking for the invisible signs none would be able to see.
You do not need to say anything to 'fix' things, simply sayng you share their grief and loss and care about them is the best anyone can do. Being company is good, isolation not so good.
Is there anyone lives near your mum might give her a hand with practical things for a while, even if it is just sweeping the kitchen. A hand with those everyday tasks can help a lot at times.
Please feel you can talk here as often as you like, it would be welcome