I don't think Pinkie can really do "mother." At least not currently. Cousin or sister, she's got in the bag. Aunt gives her trouble occasionally, but she's been practicing on the Cake Twins, so she can totally be your aunt if you want. But, much as I love Pinkie, she's not responsible enough for "mother."
Extremely sweet comic! Just today I had the thought, that Applebloom can also go to Pinkie Pie now, when she has a problem and needs somepony to talk to, if Applejack is not there at the moment. I'm excited to see which role Pinkie will play in the Apple Family from now on!
And it makes me wonder about the finale of Season 4, now that I think of it...... Can it be that one pony dies in the finale, Applejack to be precise, and that Pinkie steps in as Applebloom's big sister, and has to comfort her and help her through that hard time from that moment on?
AJ is more like the father-figure, which makes for a very akward shipping. And before you ask: Mac is the older bachelorīs uncle who sits alone somewhere in the corner of every family meeting. You know the type