I learned their names and don't let them and their crazy mom affect my household. It's up to my husband to have a relationship and do things with HIS kids. My interest in them extends to protecting myself from them. 

" so a lot will naturally fall on me since she doesn't’t have a mom "

Not necessarily. This man manages to be raising his daughter quite a lot all by his lonesome without your assistance. What he can't do, his MIL is doing (child's grandmother on her deceased mother's side). 

You've already told us you stay at your home with your own two daughters 50% of the month (EOW). You've already told us that on the weeks you don't have your own daughters that you are busy working, doing yoga and other activities that you don't do while having your own kids. You've already told us you and this gentleman have to work at finding time for each other.... so why would he be asking you to take his daughter to the mall? Why would 'motherly' things "naturally fall to you"?

Where is this idea that he will push 'motherly' things or assign any "motherly duties" to you coming from? 

You've known him a year, his wife has been deceased two years. You weren't around to do any mother type things in that in-between year. He handled his daughter quite fine with no GF in his life. 

Whether it is himself or his neighbor MIL (aka Grandma) some one besides you are doing the cooking, laundry, house cleaning, child care and "chauffeuring"  right now that isn't you. 

You're not planning on moving in with this guy any time soon as you've already said your two daughters would be upset at the idea. 

So am I to assume all this worry and fretting about all the 'naturally fall to you' motherly things are just a part of you trying to figure out the future of this relationship? 

I'm really not sure why you're asking about 'what if she asks me to take her and her friends to the mall'.  

FWIW, giving a kid a lift to the mall isn't necessarily a 'motherly thing' , but if you are asking if perhaps you are taking your two daughter's to the mall and your boyfriend's daughter ask if she too, may come along, that would be optional. Not mandatory. As the kid has gotten along fine for two years without you taking her to the mall, I'm sure her father and her can and will figure out other arrangements than burdening you with driving her there. 

And in the event you one day move in with boyfriend and the child, you already know you would not be driving the kid to any mall because you would resent the kid and also feel guilty you are taking her while your own two daughters are with their father and can not go. 

So the natural response to 'please take me to the mall' would be a round 'no, ask your Dad'. 

I also read your previous post and can give you some advice from someone who had 2 BDs and 3 SDs. When my BDs are with me, they are my responsibility 100%, now there may have been a time or two where I had to involve my DH in their care where I couldn't get to them.  Two times they were sick at school and because he works 3rd shift and is home all day, I had to call him to get them from school because I couldn't leave work.

As far as my SDs, I maybe took them to school a couple of times because of my DH working OT and couldn't do it.  Otherwise his kids as him for things and mine ask me for things.  If you move in with this guy, I'm sure your future SD won't be coming to you for anything at all.  

You have only been dating this guy for a year and to tell you the truth it sounds like this girl needs counceling to come to terms with the dealth of her mother.

Non whatsoever, I never did feel welcome and still don't so I disengaged from the beginning and I guess so did he. 

I was all in as a parent to SS-25.  My wife was the CP on our blended family adventure and with the exception of the 7wks (5wks summer, ~1wk winter, 1wk spring) of SpermLand visitation per year we had SS 24/7.   It never bothered me to be his dad, schlep he and is friends around, etc.....

My bride and I agreed early on that if we were to be equity life partners then we would also be equity parents to any  children in our home regarless of kid biology. At it turned out... my SS was an only child in our home.  His mom and I met when he was 15mos old and married the week before he turned 2yo.

Our situation worked out pretty well.  My son asked me to adopt him when he was 22. We made that happen.

So, my advice is to not settle for anything less than equity parent status regarding your Skid(s).  That does not mean that you have to be the one to do all of the parental drudgery work but certainly that can be part of it.

In my situation, my BF's son is 4 and we've been dating over a year. I technically have my own apartment, but I'm at his place 99.9% of my time. He has him 40% of the time, and unless I'm working or have occasional plans with friends, I'm involved with what they're doing. We play outside, play with toys, read books, and I join b-day parties and such. BF and I share cooking and cleaning, and I help lay out the routine, boundaries, and consequences. This is the only child between us, so it's much easier than your situation. 

IMO, I would shift your thoughts and focus on the fact that this girl no longer has a mother. You don't have to replace that role, but being a present and supportive woman in her life would be huge to her. I would think about what does that role look like to you? Accpeting this is part of accepting the man you're with. Children can definitely pick up on how you feel about them, so be aware of what that is and examine it. Asking for a ride to the mall with her friends is harmless.... but I don't know how exhausted you felt at that point from beign relied on too much. If you feel like the responsibilities of the relationship is too much too soon, then I would communicate this with your SO and spend a little more time away from his household so he handles those kinds of things. Definitely lay out clearly what your limits are so he can support you. At the same time, in order to have a more harmonious future as a blended family, spending time bonding in a positive environment helps build a good foundation for later. Good luck

I've been in this situation before (but the mother was alive and he only had his kids 50% of the time) and i did do things with his kids when he was at work (he was a firefighter and worked nights too sometimes) because i loved him and wanted his kids to care about me too if i was going to be in the picture long term.  they were only 4 and 6 at the time.  It's up to you - if you want to give it more time, then do so.  Go at your pace.  If you think this is something that you won't want to do long term since the mother is gone, then maybe you should just bow out now.  they will always be the biggest part of his life. some people have a really hard time with other people's kids - and especially if the mother is gone - that makes it more difficult and complicated.  i get it.  who are you resentful at?