Sibomana Joseph yahawe isakaramentu ry’ubusaseridoti kuwa 25 Nyakanga 1940. Yatorewe kuba umushumba wa Diyosezi ya Ruhengeri muri Kanama 1961, muri Nzeri 1968 agirwa Musenyeri wa Diyosezi nshya ya Kibungo, ayiyobora kugeza kuwa 30 Werurwe 1992 ubwo yajyaga mu kiruhuko.
Mu buzima bwe yaranzwe n’ibikorwa by’urukundo no kurwanya ivanguramoko n’amacakubiri. Bimwe mu bikorwa by’ingenzi byamuranze twavuga nko kuba yarabaye uwihaye Imana wa mbere w’Umunyarwanda utarahigwaga watinyutse ku mugaragaro kuvuga ko Leta ya Gregoire Kayibanda yica Abatutsi, arwanya ivanguramoko n’amacakubiri byari byarimakajwe n’ubutegetsi bwa PARMEHUTU. Yamaganye mu nyandiko ubwicanyi bwakorewe Abatutsi muri Byumba mu mwaka wa 1962 bugahitana abarenga ibihumbi bibiri (2000). Mu mwaka wa 1963 yavuye mu Ruhengeri ajya kureba Perezida Kayibanda i Kigali amusaba guhagarika kwica Abatutsi.
Musenyeri Joseph Sibomana yitangiye kandi uburezi budaheza, maze kuwa 5 Ukwakira 1968 atangiza Iseminari Nto ya Zaza yizwemo n’abana b’Abanyarwanda batari bafite uko biga kubera politiki mbi y’iringaniza. Mu mvururu zo mu 1973, yaharaniye ko nta Mututsi wirukanwa mu mirimo no mu mashuri bya Diyosezi yari ayoboye, ahubwo yakira abanyeshuri birukanwaga mu yandi mashuri.
Musenyeri Joseph Sibomana yari Umushumba utarangwa n’irondabwoko kandi akanga akarengane. Buri gihe yasabaga abanyeshuri, abayobozi n’abakozi ba yo kwimakaza ubumwe no kwitandukanya n’ivangura iryo ari ryo ryose kubera ko na we atigeze avangura kandi akaba yararanzwe no gutoranya abapadiri n’abarezi batabijyamo. We ubwe yashyize imbere kurera abana atabasumbanyije, arwanya ivangura aho riva rikagera, mu gihe nyamara politiki y’ubutegetsi bw’icyo gihe yavanguraga Abatutsi ku mugaragaro. Yashimangiraga ko umuhamagaro wa Seminari n’uw’abayibamo ugomba kuba uwo gushyigikira no kogeza inkuru nziza kandi inkuru nziza ntiheza, ni iya bose.
Muri Jenoside yakorewe Abatutsi, Musenyeri Sibomana yari mu kiruhuko cy’izabukuru i Rwamagana. Mu rugo rwe yakiriye abatutsi bahigwaga akora ibishoboka byose arabarinda, akajya aha Interahamwe amafranga ngo zitabica kugeza ashiriwe akajya abaha n’amafranga y’amahanga, ku bw’amahirwe Inkotanyi zihagera abo bantu bataricwa