Is Power of Attorney or Executor of Estate in charge of house and car?

I’m guessing that there are very different wills for both mom AND dad & that is the source of conflict, is that it?
And the assets of mom’s estate are freestanding AND some are also commingled with dad as to ownership, right? 
Perhaps assets being bled? My answers are based on this being the concern.....

If your named as the executor in mom’s will, then you can open probate. 
Neither dad or Sissy can stop this. But if you do, I’d highly suggest that you get a probate atty who does litigation. Most probate guys do NOT do litigation at all, never ever. So you need to clearly ask. If mom’s will was done by a different attorney than dads was, I’d suggest you contact that firm to see who they suggest for probate (with litigation) atty. I’ve been executor 3 times, regular probate guy maybe $2500-4000 but litigation probate may need a upfront retainer $5k -$7500. It’s speciality work.

You might want to roughly do a quick determination as to what mom’s assets could have been as of her DOD. Day of Death. If it’s significant and you get the vibe that Sissy or Dad is depleting mom’s assets & fast, you need to move to get probate opened quickly and have mom’s assets frozen. And the atty will get a forensic done to determine DOD assets. Your litigation guy will know exactly how to do this & how to recover assets removed. Oh also the $ for the retainer, you as executor will be repaid by moms Estate (assuming there’s $) and you won’t have to wait for the final distribution to do this. Good luck!

jbleedin, freqflyer's answer is commonly correct if your parents' house title was of the "joint tenancy with survivorship" type, but not if it was of the "tenants in common" type, except probably in "community property" states (just 9 of the 50 states, I think). The facts that your mother named you as her estate's executor and that your father gave your sister his POA may indicate that they had different preferences, which may have resulted in different actions and intents. So, unless your dad's POA, and the house title, and your mom's will are very clear and non-conflicting, you may need legal assistance if you and your sister are not able to work together, even after help from a family mediator that I suggested earlier. And based on your prior question about wanting a monthly financial statement from your POA sister, I think you and your sister might benefit from a family mediator. Hope this is helpful.

jbleedin, my heartfelt sympathy to you and your family for the passing of your Mom.

It is the Power of Attorney for your Dad who is in charge of the house and car. Sounds like everything was in joint name for your parents, so that made it very simple, like no Probate was needed [depends on your State]. Your Mom's estate went directly to your Dad, thus your job as Executor should be done.

jbleedin, who's in charge of what depends on what state your mother and father lived, exactly how their names are listed on the house and care titles, and exactly how your mother's will and your father's POA are worded. Best case is for you and your sister to be able to work together, perhaps with help from a professional family mediator and/or you may need an attorney's assistance.