I want to write a cute, slice of life, family themed fic for the pie sisters. But I have no clue how to go about doing that.

Try setting this in the future. Have Pinkie Pie come home to the rock farm for hearths warming after Maud has graduated college (or what I can assume is college). You could stop at that, but if you want, I got an add on: Maybe you could add a cheese pie side story idea where pinkie brings cheese with her and/or a foal (this why she skipped one or two hearths warming celebrations due to either her being in labor around that time or just too busy taking care of him/her or just spending time with cheese). Maybe the family doesn't know about this due to a mix up in the mail or just forgetfulness to tell on pinkie's part and the family has to try to make the baby and/or cheese familiar with their traditions and them. They end up combining cheese's traditions with theirs and the baby has an amazing first family hearths warming!

You don't have to use my idea if you don't want to

Hmm..I like your thinking here. I'm not a huge ship monger. Especially of Pinkie Sandwich...but it seems like an interesting story idea. I may elaborate and tweak it.