Sigtrygg Bengtsson (Boberg) - The mother of the daughter of Magnus Minneskold who married Sigtrygg Bengtsson (Boberg) was almost cetainly Ingrid Ylva.

The second wife of Sigtrygg’s Bengtsson Boberg was a halfsister of Birger Jarl, they had the same father, thus different mothers, thereof born before her father Magnus marriage to Ingrid, his first wife died in the late 1100's,

Magnus children in his first marriage

Eskil Magnusson, b. ca. 1170-1175, d. ca. 1227. Lawman in Västergötland.
A daugher married to Sigtrygg Bengtsson (Bobergs ätt).

Children in his second marriage with Ingrid Ylva. (d. ca.1251)

Karl Magnusson, bishop in Skara 1216-1220, ought to be moved to his first marriage, as he most likely was born between 1180-1190, or he became a bishop as a child...
Bengt Magnusson, Bishop in Linköping 1220-1236, d. 1237.
Birger jarl, regent

Firstly, forget this hook up on the precise years, in most cases this far back the years, if not explicitly written down in a source, are just estimated values, and we do have a need for them in order to prevent the most ridiculous merges that people unfortunately otherwise will do, merging people from different centuries just because they have similar names, with nothing to show them how far out from reality the're without this red warnings that show some kind of discrepancy, so Sigtrygg Bengtsson (Boberg), wasn't actually born 1197, he was born before that year, but not after that year.

Secondly, I will have to eat quickly, get to work, and continue this part when I'm back.

Secondly, the marriage of Ulfhilds son Magnus daughter Ingrid to Bengt Boberg, grandson of Ulfhilds brother, supports the idea that Bengt Sigtryggson (Boberg) actually was a halfsibbling to Ulfhild, otherwise they would have needed a permission from the pope to get married, but it doesn't end there, Ingrid Magnusdotters mothers father was a son of Elof Magnusson , halfbrother to Birger Jarl, they had the same mother, not the same father, thus Elof father was unknown, (unfortunately, some people here seem to like incest and has set up Magnus as his father once again), but this at the other hand, support the idea that Ulfhild wasn't Ingrids granddaughter, what we get is a more likely scanario, with two unknown first wives of Sigtrygg Bengtsson (Boberg), the first one he had the son Bengt, the other one he had the daughter Ulfhild, before he married a third time with Magnusdotter.

Bengt Larsson (Boberg)
Ingrid Magnusdotter (Lejonbalk)

https://www.adelsvapen.com/genealogi/Lejonbalk#TAB_2

1. "second cousin", = "syssling" in our terms, and yes, that would have been two close according to the roman catholic computatio canonica. At the other hand, if they were just half-second cousins, it might have been slipped away under the chairs, as we say here.

2. Birger Jarls remains have been analyzed and that result gave his birth year to be around 1210, so his father died between 1208-1210, not much to debate, Elof is accepted as the son of Ingrid Ylva, and as his fathers name are unknown combined with the fact that his children used another coat of arm, it's common accepted that he only was a halfbrother to Birger and not placed under Magnus as his child, and still, the daughter married to Boberg is placed under the first wife, not the second one. Karl, as I mentioned earlier, have not been placed under either of them, because of his occupation as bishop 1216-1220 that suggest that he was much older, thus likely not the second wifes child. https://sok.riksarkivet.se/sbl/bilder/14301_8_016_00000262_0.jpg

3. The vast majority of the nobility remarried regardless their gender, that's a fact and they did it quite fast!