How to succeed?

Thank you for reaching out via BetterHelp with this question. I think it's a really difficult situation to be in, when we feel like the actions of others are impacting our own mental health. We can't change them, or their behaviours, but we can work with how we respond, and how we maintain and reinforce our own boundaries.

Boundaries

It can feel really difficult to set boundaries to keep ourselves OK, and still be flexible with those boundaries when we have the energy and capacity to help those around us. 
If this was going to be your main therapy goal, I would expect you and your therapist to explore what boundaries you feel comfortable sticking to, and which boundaries you struggle enforcing. What does enforcing a boundary feel like, and what happens when those around you don't like it?

'if I'm not there for them who will?'

This feels like a lot of pressure for you to be under, especially because they're your nephews. Some families have great support networks (it must be nice to be part of one of those families!) however some families are very small and thinly spread. I would be asking: Why do you have to be the mom that they need?

This is a topic I would explore within the therapy, personally, looking at what you're able to give without burning yourself out.

We cannot pour from an empty cup, and it sounds like you're looking out for not just yourself, but your sister and your three nephews. That's a lot of people for you to be thinking about, holding in mind and supporting on a daily basis.

Successful vs Stuck

I totally support and commend you for wanting to try therapy and seeing how you can focus on your own success. This often has a trickle down effect on the people around us, in our lives, when we're working through some of our own content, it can make it easier to deal with, or recognise these things, in others. 

Looking at what your goals in your life are, and how you want to support them and continue to work towards them might help lessen that feeling of being stuck. That feeling of making progress, however small, can be a huge game changer.

What next?

If this is something you decide you want to work through with a therapist, they'll be able to explore the options and you'll likely set a therapy goal together.

Not all therapists are right for all clients, so, I would suggest making sure you feel safe with the therapist you end up working with. The great thing about BetterHelp is that if you don't feel like your first therapist is a good fit, you can change until you find someone that you feel does fit, and will work with you in the way you want.

Whatever path you choose, I wish you all the best with this work. It sounds like it could be a lot of layers of deep work and self awareness.

Best wishes, 
Jess