So now family tree of Lesser Poland's Piasts looks like this:
Bolesław V (1226-1281) Duke of Cracow 1243-1281 m. a) Kinga of Hungary (1234-1259) b) 1261 Eufrozyna of Greater Poland (1247-1298)
1b) Leszek II (1264-1330) Duke of Cracow from 1281, Duke of Greater Poland and Pomerelia from 1296, King of Poland 1300-1330. m.1278 Kunegunda of Bohemia (1265-1321)
1) Bolesław VI (born 1283). m. 1303 Elisabeth of Austria (born 1285)
2b) Bolesław (1268-1269)
3b) Przemysł (born and died 1270)
4b) Elżbieta (1273-1325) m. 1290 András III (1265/70-1301) King of Hungary and Croatia 1290-1301
1) István VI (born 1292) King of Hungary and Croatia
2) Erzsébet (born 1294) m. 1306 Václav III (1290-?) King of Bohemia 1305-?
Not all children are mentioned in third generation (some would be added). I think being nephew of Przemysł II is enough and Leszek would not need cousin marriage for his son to strenghten his claim even further, daughter of King of the Romans is politically more profitable match IMHO.