Papyrus and Sans would 100% be BFFLs with Mario and Luigi. (However, I do think Sans favors Luigi a bit more b/c he's gentler - Mario may be a bit too aggressive sometimes for Sans's taste).
1) Mario and Papyrus go on hikes/runs together (Mario would never let Papyrus join him on an actual adventure though; he's overprotective with him as he is with Luigi (they both HATE that btw))
2) Papyrus and Luigi share pasta recipes with each other have have cooking competitions; Mario and Sans are the judges
but they each vote for their own brother, so another friend has to come in and be the tiebreaker
3) Whenever M&L come to visit their world, they bring star bits with them!!
4) Sans and Luigi conducting experiments on the powerups and just being science nerds in general
5) Mario drags Sans into an ongoing "Best Big Brother" competition
6) Sans occasionally falls asleep on Mario; his fire powers makes him warm
7) Papyrus and Undyne helping Luigi become more confident in himself :] (They literally shout words of encouragement at him)
Sans/Mario and Papyrus/Luigi are also really similar to each other too,,,
When I get my thoughts in order, I may expand a bit more on that,,, BUT DO NOT UNDERESTIMATE MY ABILITY TO MERGE MY TWO FAVORITE THINGS TOGETHER /LH /SRS