It is completely UNreasonable to expect a stepparent to pay anything for a skid. That is the responsibility of the bioparents. It is the choice of a stepparent to spend money on a skid OR play skidsitter (which I seriously do NOT recommend).

There are ZERO reasonable expectations of step-parent.
Anything a step-parent GRANTS to the bio-parent or skid is up to the step-parent.
If there are "expectations", you are already on the backfoot.

Financially, I think expecting a stepparent to pay anything toward a stepkid is unreasonable UNLESS the stepparent insists on a standard of living the bio parent cannot afford and expectations are discussed ahead of time.  Example: If stepparent insists on living in an expensive house that the other parent couldn't otherwise afford, it would not be unreasonable to expect the stepparent to pay for more than his/her proportionate share of the housing expense.

As for other things, I think a lot depends on the family and the circumstance.  In my household, if my husband was the only one working outside the home so that I could stay home with our bio (which is our plan for when we have a bio), it would be reasonable for him to expect me to get the skids onto the bus in the morning when they're with us and to be responsible for them until he got home.  This would be reasonable, in OUR situation, because (1) he couldn't afford additional childcare expenses on top of providing all support for the family, (2) skids treat me well and we operate as a family unit when skids are in our home, and (3) I have always had authority to go along with responsibility for the skids.  In another family, expecting a stepparent to be responsible for skids might be completely unreasonable.

The only think I think is expected is that you will be reasonably kind and civil to the kid - provided the kid does not treat you like dirt.  Otherwise, anything else is all voluntary.

I do everything for my stepdaughter that I do for my son. In fact, my husband is a SAHD with our son/my stepdaughter. But my situation is unique and I am the only involved mother she has and my husband backs me FULLY (and so does BM) as her "mom" 

In an average situation, the only reasonable expectation of a stepparent is to treat your child kindly and let you parent as needed ie. not say "you can't see SD/SS today because I want to hangout with you alone" and crap. 

Any expectations beyond that are ridiculous. 

There no rules.  Just remembered, you were not asked if your DH should of had kids with BM.  It was DH and BM choice to have kids, they should raise them.  You do not have to do anything for SK that you would not do for anybody. You do not have to pay for them. Babysitter sit them, ect.  That BM and BF to figure out. 

If DH is not home, SK are not home either. They are either with, him BM or he arrange child care someplace else 

Your responsibility as a step parent is to be nice and respectful to his kids and accept the fact that he is their father and needs to be there for them at times when you would like him to be available for you(school, important events sickness, emergencies, etc). If his kids are disrespectful or rude towards you it is ok for you to disengage until DH deals with it appropriately. Do not put up with disrespect however do not retaliate by being cruel towards his children in return. If he does not deal with his child's behavior appropriately that is a red flag. It is not your responsibility to pay for his kids nor play free nanny or sitter to his kids. They have two parents who should be doing this. That is their responsibility, not yours. Do not be used. You should be in a role of a partner, not a mother, as those are not your children and never will be. Do not forget that. You have no financial obligations to those children whatsoever, legally or otherwise. Anytime you choose to spend money on those kids it is your choosing to do so, not your obligation. Remember that.

Is being civil and honest.  Not interfereing with the money I have saved for my DC for college. 

and no bio parent should marry someone quickly and alienate their kids saying you have a new mum or dad etc.

it is unreasonable to expect stepparent be a free babysitter, it is unreasonable to demand they spend time with your kids like they’re the centre of the universe

BM told me to "know my place", so that's my go to line when I dont feel like being a doormat, maid, babysitter, chef...etc. we have 2 young kids together anyway, my goal is to be their mother and look after them. If they want someone to go give them money or take them places they can walk over to her house. Essentially it's the CS my husband pays her which pays for all the wonderful shit she buys them. At least my children wont grow up spoilt. 

The expectations are and should be entirely on the BioParent and Skids.  They do what the SParent dictates or their keys for the home quit working.

Fortunteately in quality marriages the spouses are on the same playing field and put each other first so keys rarely need to be found to be ineffective.

My best advice to anyone only just considering cohabiting with someone with kids is to have the talk first, ask them what they expect and tell them what you're prepared to do, saves years of frustrations . 

In my case it's been a long road from being expected to play 2nd mummy and silently tolerate everyone's crap, to all I do these days, which is be polite, give ss a hot meal, that's it. Totally disingaged. 

I think of it as Hubby and I are a team. I knew he had kids before we married and a CRAZY BM.  The kids need both parents and need to know if divorce happens its not the end of the world as some BM's make it out to be. Look at it as now they have more people to love them. its not a competion w/the kids, yes issues arrise and feelings get hurt. Dont cross certain lines or make decsions be there for support of your husband and know that kids are only young once you plan on having a lifetime with your hubby.  If asked to watch the stepkids, make it fun then it wont feel like its an unreasonable action.  also set limits...such as is this a constant thing of having to babysit and why?  Communicate...Communicate...Communicate.  I always make SKids ask a parent if something is ok, such as movie, certain clothing you know basic things...you also tend to earn more respect from BM (if thats possible LOL ..maybe not LOL esp. in my case) your not trying to play parent your just keeping things "respectable but not akward"  and never ever ever question the kids or make comments about the other parent. NEVER. good luck to you! 

My three stepkids have stolen several thousand dollars worth of things I owned before I met their mother, consistently and constantly lied to her and I over the smallest and largest of things, gotten me 8 days in jail that also came with 2.5k in fines and 150hrs community service...all for the youngest two being thieves. Did I miss the chapter that gives the table for dollars lost or stolen and lies told to you before you leave? Is there a metric out there for that kinda thing?