Before anything we need to establish size of each of our contenders. Based on official data Guts is around 6'8" while Peppa is at whooping 7'1''. That makes her little brother George 4'6'' and Daddy and Mommy pig 15' and 12'2'' respectively.
We've seen Guts face opponents who are much bigger than him before and still come out on top, but now he needs to do that 1v4. In those situations he often uses his cannon combined with the Dragonslayer to strike more foes at once. However, will it be of use this time?
It's safe to assume Guts could take down George and even Peppa from distance using his crossbow and cannon but real problem to him are Daddy and Mommy Pig who are double his size. Note that he had issues fighting Grunbeld without Berserker armor, and he's as tall as those two.
Even considering all that, Guts could probably take down all 4 without bigger issues knowing he's a master swordsman and none of the Pig family is known for owning any weapons or knowing martial arts. We can't really say they could hurt Guts in any way aside from stepping on him.
However, that all changes if we assume Peppa actually has a secret weapon of her own. During Easter special Peppa is seen with what looks like a Behelit. Could it be that she was secretly an apostle all this time? Then we can only imagine how scary her true form actually is.
During Halloween special Peppa is also seen dressed as a witch. We didn't see her use any spells but if she really could use them, it would be a massive problem for Guts. Imagine Schierke only 3 times taller wielding dark magic and you got yourself a problem.
Another member of the Pig family known for his magic is Daddy Pig. He's seen summoning creatures from his hat which we can only assume is a portal to the Astral World. This could be a huge problem as well, because number of opponents Guts would have to face could be endless now.
This all would probably leave Guts forced to use the Berserker armor. Considering Schierke couldn't help him, we don't know if Beast of Darkness would fully take over. By the end of this battle he would probably be destroyed. So yeah ... he could probably win, but at what cost?