I travelled to my house and got the enemy "Never should have come here" screams. I saw the little red dots but couldn't find them. I used tcl to discover they were in the interior of my house (but I was outside,so they were standing in a blank room because I hadn't loaded the inside)

When I came in,they all attacked me,my family,my housecarl and my bard. They were bandits added by OBIS,so I don't know if this is a weird mod thing,or normal for hearthfire?

M'aiq thinks you should of not stole their sweetroll...

M'aiq thinks that is normal for Hearthfire.

Never spawned inside my home, but they did follow me inside.

It's not normal in the vanilla game, no. I've seen lots of bandits respawn outside of houses, but never inside. You're on PC and have admitted to using some mods. I'd look into those mods if I were you. Is it possible to disable that mod you mentioned, make a save, start the game up again, and see if those bandits are still in there?

I've had it happen with creatures clipping inside the walls and I had to tcl to kill them. They never showed up inside the house cell though.

I believe its possible for them to spawn with you if you fast travel there and then load with you inside the house. Or were hostile to you outside but you didn't notice and they followed you in.

but actually spawning inside the house, I don't think that was meant to be so.

As others have said, it isn't normal for this to happen, but it still does occasionally. Unfortunately, this is an engine bug and there is no way to go about fixing it.

It happened to me ONCE at Lakeview Manor in many, many, many hours of gameplay. I rode up to the house expecting the usual outside spawn, heard the threats from the bandits, but couldn't find anyone anywhere. I turned on my HUD and saw three dots on my compass, but still couldn't find anyone. So I went inside. Yep, the bandits were in the house. However, since Rayya, Lydia, Serana, and Bran live there and I had J'zargo and Derkeethus with me when I stepped inside (all of us over level 60 thanks to AFT), the poor bandits didn't stand a chance.

Those OBIS bandits are sneaky bastards.

I've never had bandits spawn inside my heartfire house. I've had some spawn fairly close by outside the house though.