I'd call the person in charge of the slide show and tell her what is going on. Personally, I believe those pics OUGHT to be included. If some pictures of the child from an earlier relationship can be found, include them in the slide show, even if it seems to be strange for them to be in there. Of course, there are later pics of her children in that slide show. Your SIL is ignoring THAT, it appears. If I were in charge of that slide show and either the brother or SIL came crying to me, I would say all of this to them and that they aren't making sense.
If I were you, at the funeral I would have someone read out in front of her and the whole funeral party who ALL deserves thanks for taking care of mom in her final days. If your husband can stand up and read that, it would be better.
Other than this, ignore her theatrics. Forget about clearing the air. If you start getting lectured, get up and walk out. Your husband is in a difficult position with his brother, but it is up to him to say "Hey, bro, we're ALL grieving over mom. You and your wife aren't the only ones with sadness, you know. EVERYONE deserved to have pictures in the slide show. Your family had plenty of photos displayed, and you should just let this go. And you WERE thanked, by me several times, even before you started complaining about it." ONE apology that they are feeling like this, and then no more apologies, because after the first one, any beating you all up over this just becomes abusive.
Ignore the issue of them only helping once they moved in. It doesn't help to dwell on it. And don't talk about it with ANYONE, or it will come back to bite you in the butt.
I'm so sorry about your mother in law.
Dawn