how to get custody of my sisters kids when dfs has taken them and refuses to give the kids to me

If you're prepared to show that you're both willing and capable of providing a good home for the children, you'll need to file a motion to the court. I very much doubt that you can get very far with this, without a good family court lawyer.

It's hard to give you much information, because there is a lot of factual background that isn't in your questions, and a lot of it matters, things like your age and situation, your sister's age and why dfs took the children, how many children and how old, what issues they might have, where's dad (that's a big one) and so on.  If you're only looking to provide temporary care, for how long?  That's just getting warmed up.

For reliable advice about your options, based on all the unique facts of your case, please talk to a qualifield lawyer in your area.  One place to family court attorneys is our website, http://attorneypages.com